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

[cliffhall]

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.

[cliffhall]

Triage: Priority Low (total 4)

• Severity 2 — the mirrors work; carrying them costs clarity, not correctness (same reasoning as Remove ext-apps SDK-v1 shims when the package ships a v2 peer (ext-apps#702) #1745)
• Urgency 2 — blocked upstream on ext-apps#733, which is still open; nothing can move until a release contains it
• Bonuses: none — milestone bonus not claimed at file time; not parented to chore(deps): upgrade the MCP Apps extension SDK to 2.0.0 #2309 (see the body for why)

Board: #28, Status Todo, milestone v2.8.0 because it plainly cannot make v2.7.0 (due 2026-09-16).

[cliffhall]

Re-score after re-parenting (maintainer moved this under #2309 and into v2.7.0): +1 sub-issue of a larger epic → total 5, still Low. Unchanged on the board. Still blocked upstream: the only published ext-apps release is 2.0.0 and ext-apps#733 remains open, so this holds #2309 open until a release containing #733 ships.

[cliffhall]

Moved back to v2.8.0 and unparented from #2309 after discussion: #2309 closes with the 2.0.0 bump, and this waits on its own upstream trigger. Score stays Low (total 4 without the sub-issue bonus).