Drop the ext-apps#733 elicitation mirrors when the package ships a release containing it #2360

Surfaced while doing #1745 / #2309 (PR #2358). Same shape as #1745 was: a set of in-source mirrors carried until an upstream @modelcontextprotocol/ext-apps release makes them redundant.

Trigger

Close this when @modelcontextprotocol/ext-apps publishes a release that contains ext-apps#733 (MCP Apps-rendered elicitations, SEP-3118). 2.0.0 does not include it — #733 (opened 2026-07-29) is still open upstream as of 2026-09-13, and 2.0.0 shipped 2026-09-08 without it — so there is no bridge.requestElicitation(), no elicitation key in McpUiHostCapabilities, and no /server helpers to switch to yet. The nightly SDK watch (#1063) will file the release issue; this is the cleanup it unlocks.

Mirror sites to remove (verified against the #2358 tree)

Each is tagged in-source with ext-apps#733:

1. core/mcp/appElicitation.ts — mirrors the add support for proxy base path routing #733 server-capability / URI validator helpers; replace with the package's /server exports.
2. clients/web/src/components/elements/AppRenderer/requestAppElicitation.ts — hand-issues the ui/elicitation request; replace with bridge.requestElicitation(params).
3. clients/web/src/components/elements/AppRenderer/appCapabilities.ts — the WeakMap + transport observer that captures the app's initialize capabilities; delete the module and read getAppCapabilities() off the bridge.
4. createAppBridgeFactory.ts ~line 326 — the Partial<McpUiHostCapabilities> spread that adds the elicitation key the type does not yet declare.
5. docs/test-servers.md ~line 246 — the note that the Inspector speaks the add support for proxy base path routing #733 wire protocol but does not yet consume its helpers.

Folded in

While touching (3): type MessageObservable / observeAppCapabilities against the SDK v2 Transport interface, which PostMessageTransport now implements (the test fakes would need start/send/close). Cosmetic on its own; do it here rather than as a separate change.

Acceptance

• Bump @modelcontextprotocol/ext-apps to the release containing add support for proxy base path routing #733 (root package.json only).
• Remove mirrors 1–5; the app-rendered elicitation flow (mcp-app-elicitation test-server config, smoke-web-elicitation.mjs) still passes.
• npm run local:gate green.

Why not a sub-issue of #2309

#2309 tracks the 2.0.0 upgrade, which #2358 completed. Parenting this to it would hold that issue open until an ext-apps release that does not exist yet, so it is a sibling with cross-references (maintainer-confirmed).

Priority scored with the /issue-triage rubric below.