Extensions: advertised-extensions toggle in Server Settings (Phase 2 of #1633) (#1743)

* Extensions: advertised-extensions toggle in Server Settings (Phase 2 of #1633)

Adds the debugging knob from #1633: users choose which extensions the
Inspector advertises to a server, and a server may register different tools
in response. Builds on Phase 1's registry + advertisedExtensions option (#1738).

Settings + persistence (core):
- InspectorServerSettings.advertisedExtensions + StoredMCPServer field, wired
  through serverList's read/write converters and INSPECTOR_FIELD_KEY_MAP
  (omit-when-empty, byte-stable round-trip).
- validateSettings (remote PUT route): isBooleanRecord guard + carry a
  non-empty map; unknown/malformed rejected.
- App.tsx threads savedSettings.advertisedExtensions into the client options.

UI:
- ServerSettingsForm renders an "Advertised Extensions" group of checkboxes
  driven by ADVERTISABLE_EXTENSIONS; a per-server override wins over the
  registry default. Modal folds a toggle into settings.advertisedExtensions.

Test server + proof:
- composable-test-server extensionGatedTools: a tool registered disabled and
  enabled on `initialized` iff the client declared the mapped extension
  (getClientCapabilities). Legacy stateful leg. Threaded through
  load-config/resolve-config; advertised-extensions-http.json showcase + README.
- Integration test connects legacy with/without the tasks extension advertised
  and asserts the gated tool appears/disappears in tools/list — the acceptance
  criterion, proven end to end, not asserted.

Tests: serverList round-trip (non-empty/absent/empty), ServerSettingsForm
toggle rows, ServerSettingsModal fold, validateSettings accept/reject, and the
integration gating test. npm run ci green (per-file ≥90 gate held).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

* Extensions: add #1739 proof screenshots (advertised-extensions toggle)

End-to-end smoke test in the web client against the legacy
advertised-extensions-http.json test server (port 3220): connecting with the
Tasks extension advertised (default) lists echo + get_weather, unchecking the
Advertised Extensions → Tasks toggle and reconnecting lists only echo — the
server's tool registration changing on the client-declared extension.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

* Extensions: reconverge advertisedExtensions to no-override on default (review)

Address Phase 2 review observation #1: toggling an extension back to its
registry default now deletes the override key (and drops the whole map to
undefined when it empties), so the on-disk `advertisedExtensions` reconverges
to the byte-stable "no override" state instead of persisting a redundant
`{ key: <default> }`. Matches the omit-when-default policy the other settings
use. Only a value that differs from the default is persisted.

Adds a modal test asserting the reconvergence (re-checking the sole disabling
override drops advertisedExtensions to undefined).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -118,6 +118,8 @@ A streamable-HTTP server can also serve the **modern (2026-07-28) protocol era**
 
 `test-servers/configs/pagination-http.json` is the **page-by-page fetch showcase** (#1721). It serves 12 tools, 12 resources, and 12 prompts (presets `numbered_tools` / `numbered_resources` / `numbered_prompts`, `count: 12`) with `maxPageSize` of 4 for each, so every list paginates into three pages. Turn on **"Fetch Lists One Page at a Time"** (Server Settings — the `paginatedLists` setting, or the **Paginated** switch in a list sidebar) and the Tools/Resources/Prompts lists load page 1 only (4 items) with a **Load next page** control and an _N pages loaded_ status; each click fetches the next 4 and appends them, and Refresh resets to page 1. With the switch off (the default), the same lists auto-aggregate all three pages on connect.
 
+`test-servers/configs/advertised-extensions-http.json` is the **advertised-extensions showcase** (#1739). It serves `echo` (always) and a `get_weather` tool that is **gated on the `io.modelcontextprotocol/tasks` extension** (`extensionGatedTools`): the tool is registered but starts disabled, and the server enables it on `notifications/initialized` only when the connected client declared that extension in its `capabilities.extensions`. Connect (the Inspector advertises the Tasks extension by default) and the Tools list shows both `echo` and `get_weather`. Open **Server Settings → Advertised Extensions**, uncheck **Tasks (io.modelcontextprotocol/tasks)**, and reconnect: the client now advertises no extensions, the server never enables `get_weather`, and the Tools list shows only `echo`. This demonstrates the debugging knob — a server legitimately changing tool registration on what the client advertises. (Legacy stateful leg only; the modern per-request leg has no persistent `oninitialized`.)
+
 `test-servers/configs/logging-legacy-http.json` and `test-servers/configs/logging-modern-http.json` are the **logging era-fork showcase** (#1629). Both serve `logging: true` plus a `send_notification` tool that emits a `notifications/message` at a chosen level; the legacy one is a plain streamable-HTTP server (`logging/setLevel` era) and the modern one sets `transport.modern: true`. Connect to the legacy server and open the **Logs** tab to get the session-scoped **Set Active Level** selector + **Set** button; calling `send_notification` streams the log into the panel. Connect to the modern one with **Protocol Era = Modern** and the same tab instead shows the **Log Level per Request** control — pick a level to opt in and the client stamps `_meta["io.modelcontextprotocol/logLevel"]` on every subsequent request (verify in the Network tab's request body); calling `send_notification` then streams the log into the panel over the request's SSE response. Set the control back to **Off** and the same call is silently gated — the request omits the `logLevel` key, so the log never arrives. That gating is faithful to the spec ("a server MUST NOT emit `notifications/message` for a request that didn't opt in") because `send_notification` emits through the SDK's request-scoped, threshold-aware `extra.log` (`ctx.mcpReq.log`): on the modern leg it reads the per-request `logLevel` opt-in from the request envelope and drops the message when the client didn't opt in or the level is below the requested severity; on legacy it honors the session level from `logging/setLevel`. Because it emits through the request's `notify`, the modern response upgrades to SSE and the log rides the originating request's stream.
 
 `test-servers/configs/subscriptions-legacy-http.json` and `test-servers/configs/subscriptions-modern-http.json` are the **resource-subscription era-fork showcase** (#1630). Both serve three `numbered_resources` with `subscriptions: true`; the legacy one also serves an `update_resource` tool, and the modern one sets `transport.modern: true`. Connect to the **legacy** server, open a resource in the **Resources** tab and click **Subscribe** — the client sends `resources/subscribe` (Network/Protocol view) and the Subscriptions section lists the URI with no stream chrome; call `update_resource` with that URI and the server updates the content and emits `notifications/resources/updated`, stamping the subscribed tile's last-updated time. Connect to the **modern** server with **Protocol Era = Modern** and the same Subscribe instead sends **`subscriptions/listen`** (its filter carries `resourceSubscriptions` + the `resourcesListChanged` opt-in) and resolves on `notifications/subscriptions/acknowledged`; the Subscriptions section then shows the stream-status badge (`Connecting…` → `Listening`) in its header, and if the long-lived stream drops it reconnects by re-listing. The modern config deliberately **omits** `update_resource`: the SDK's modern leg is stateless/per-request (`createMcpHandler(() => createMcpServer(config))`), so the tool would run against a throwaway server instance — the content change wouldn't persist for the next `resources/read`, and its `resources/updated` wouldn't reach the (separate) listen stream — which is more confusing than useful. The live update-notification round-trip is therefore demonstrated on the legacy (stateful-session) server; the modern server is for the subscribe/listen/badge behavior. (The Inspector's _receive_ path is era-transparent, so a real stateful modern server that routes `resources/updated` onto the listen stream drives the subscribed tile the same way.)