Era-aware connection model & capability gating (#1626) (#1697)

* Era-aware connection model & capability gating (#1626)

Make protocol era (SEP §7.8) a first-class, per-server connection dimension,
orthogonal to the transport type, and drive the SDK Client's versionNegotiation
from it.

Config
- Add `ServerProtocolEra` ("legacy" | "auto" | "modern"), `DEFAULT_PROTOCOL_ERA`
  (legacy, per the SDK's don't-auto-probe guidance), `MODERN_PROTOCOL_VERSION`,
  and `eraToVersionNegotiation()` in core/mcp/types.ts.
- Persist `protocolEra` on `StoredMCPServer` / `InspectorServerSettings`,
  round-tripping through serverList.ts (omitting the default on disk) and the
  `/api/servers` read + write validators.

InspectorClient / state
- Thread `versionNegotiation` through `InspectorClientOptions` (default legacy),
  replacing the hardcoded pin. After connect, read `getProtocolEra()` /
  `getDiscoverResult()` / `getNegotiatedProtocolVersion()`, expose them via
  getters, dispatch `protocolEraChange` / `discoverResultChange`, and clear on
  disconnect. `useInspectorClient` surfaces `protocolEra` / `discoverResult`.

Web UI
- ServerSettingsForm: a Protocol Era selector bound to the per-server setting.
- ConnectionInfoContent: era-aware — Era badge, Session ("Sessionless" on
  modern), and a Discovery section (supported versions + extensions) from the
  `server/discover` result.

CLI/TUI also map the mcp.json `protocolEra` into versionNegotiation so the field
applies uniformly. Version negotiation stays fully client-side; modern connect
works over the existing remote tunnel and disconnect is sessionless (the SDK
skips DELETE when there is no session id). Mcp-* header fidelity is left to the
History/Network card.

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

* Address @claude review: era docstrings, era validation, stdio session label, AGENTS.md rule

- Fix stale docstrings/comments in inspectorClient.ts, useInspectorClient.ts,
  inspectorClientEventTarget.ts, and ConnectionInfoContent.tsx: getProtocolEra()
  returns "legacy" (not undefined) on a plain legacy connect; it is undefined
  only when not connected.
- Validate protocolEra at the storedFieldsToInspectorSettings read boundary
  (new shared isProtocolEra guard) so a hand-edited mcp.json read directly by
  the CLI/TUI drops an unknown era instead of passing it to
  eraToVersionNegotiation. Keeps the exhaustive switch intact.
- ConnectionInfoContent: the Session row is HTTP-only — show "N/A (stdio)" for
  stdio instead of the misleading "Session-based".
- AGENTS.md: add a rule against unjustified `as unknown as` double casts, and
  drop the one such cast introduced in the serverList test.

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

* Address re-review: dedupe isProtocolEra guard in remote server

Replace the local isProtocolEra in remote/node/server.ts with the shared guard
now exported from serverList.ts (already imported here), so the era literal set
lives in one place and can't drift.

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>

core/mcp/inspectorClientEventTarget.ts

@@ -34,6 +34,8 @@ import type {
   Task,
   CallToolResult,
   ProtocolError,
+  ProtocolEra,
+  DiscoverResult,
 } from "@modelcontextprotocol/client";
 import type { SamplingCreateMessage } from "./samplingCreateMessage.js";
 import type { ElicitationCreateMessage } from "./elicitationCreateMessage.js";
@@ -56,6 +58,10 @@ export interface InspectorClientEventMap {
   serverInfoChange: Implementation | undefined;
   instructionsChange: string | undefined;
   protocolVersionChange: string | undefined;
+  /** Protocol era negotiated after connect (SEP §7.8); `"legacy"` on a legacy connect, undefined when not connected. */
+  protocolEraChange: ProtocolEra | undefined;
+  /** `server/discover` result on a probed/pinned connect; undefined on legacy. */
+  discoverResultChange: DiscoverResult | undefined;
   message: MessageEntry;
   stderrLog: StderrLogEntry;
   fetchRequest: FetchRequestEntry;