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/__tests__/fakeInspectorClient.ts

@@ -8,9 +8,11 @@
 import { vi } from "vitest";
 import type {
   ClientCapabilities,
+  DiscoverResult,
   Implementation,
   LoggingLevel,
   Prompt,
+  ProtocolEra,
   Resource,
   ResourceTemplateType as ResourceTemplate,
   ServerCapabilities,
@@ -45,6 +47,8 @@ export interface FakeInspectorClientOptions {
   serverInfo?: Implementation;
   instructions?: string;
   protocolVersion?: string;
+  protocolEra?: ProtocolEra;
+  discoverResult?: DiscoverResult;
   serverSettings?: InspectorServerSettings;
 }
 
@@ -58,6 +62,8 @@ export class FakeInspectorClient
   private serverInfo: Implementation | undefined;
   private instructions: string | undefined;
   private protocolVersion: string | undefined;
+  private protocolEra: ProtocolEra | undefined;
+  private discoverResult: DiscoverResult | undefined;
   private serverSettings: InspectorServerSettings | undefined;
   private appRendererClient: AppRendererClient | null = null;
   private sessionId: string | undefined;
@@ -153,6 +159,8 @@ export class FakeInspectorClient
     this.serverInfo = options.serverInfo;
     this.instructions = options.instructions;
     this.protocolVersion = options.protocolVersion;
+    this.protocolEra = options.protocolEra;
+    this.discoverResult = options.discoverResult;
     this.serverSettings = options.serverSettings;
   }
 
@@ -180,6 +188,14 @@ export class FakeInspectorClient
     return this.protocolVersion;
   }
 
+  getProtocolEra(): ProtocolEra | undefined {
+    return this.protocolEra;
+  }
+
+  getDiscoverResult(): DiscoverResult | undefined {
+    return this.discoverResult;
+  }
+
   getServerSettings(): InspectorServerSettings | undefined {
     return this.serverSettings;
   }
@@ -259,6 +275,16 @@ export class FakeInspectorClient
     this.dispatchTypedEvent("protocolVersionChange", protocolVersion);
   }
 
+  setProtocolEra(protocolEra: ProtocolEra | undefined): void {
+    this.protocolEra = protocolEra;
+    this.dispatchTypedEvent("protocolEraChange", protocolEra);
+  }
+
+  setDiscoverResult(discoverResult: DiscoverResult | undefined): void {
+    this.discoverResult = discoverResult;
+    this.dispatchTypedEvent("discoverResultChange", discoverResult);
+  }
+
   setAppRendererClient(client: AppRendererClient | null): void {
     this.appRendererClient = client;
   }