fix: resolve stream ID from event store metadata in InMemoryEventStore - #3149

Fixes #2560

Root cause

InMemoryEventStore.getStreamIdFromEventId derived the stream ID by parsing the
event ID (eventId.split('_')[0]). Event IDs have the form
${streamId}_${timestamp}_${rand}, which is not a reversible encoding when the
stream ID itself contains underscores:

• _GET_stream (the standalone GET stream) → parsed as ''
• stream_1 → truncated to stream

Impact (measured)

With a real WebStandardStreamableHTTPServerTransport + this store, resuming a
standalone GET stream via Last-Event-ID returns 200 text/event-stream but the
stream is closed immediately (0 ms) with zero frames, and the successor stream is
never registered under _GET_stream — every later server-initiated notification
on that connection is silently dropped. The default client retries
(maxRetries: 2) and then gives up. Replaying already-stored events is also
empty, because replay filters under the wrong stream ID.

Scope note: _GET_stream is the only underscore-leading stream ID used by the
shipped transport (request streams use crypto.randomUUID()); the truncation
path only affects direct users of this exported example store whose stream IDs
contain underscores.

Fix

Read the stream ID from the store's own metadata instead of parsing the event ID:

return this.events.get(eventId)?.streamId ?? '';

The store already persists { streamId, message }, so this is the single
unambiguous source of truth (the "explicit eventId→streamId mapping" option from
the issue). String parsing — including lastIndexOf — cannot work in general,
because the random suffix may itself contain underscores.

Behavior change

• replayEventsAfter now returns the correct stream ID and replays subsequent
  events for _GET_stream and any stream ID containing underscores.
• Unknown lastEventId / empty store still return '' (unchanged).
• No interface or signature changes; the EventStore contract is untouched.

Testing (both sides)

• RED (base implementation + the new tests verbatim): 2 failed / 2 passed —
  expected '' to be '_GET_stream', expected 'stream' to be 'stream_1'.
• GREEN (with fix): 4/4 new tests pass; package suite 2 files / 6 tests pass;
  build:all, lint:all, pnpm -r typecheck, transportResumability (3/3),
  hosting-resume e2e (14/14) all pass.
• Transport-level probe (real transport + real store): base → resume closed at
  0 ms, no frames, mappingKeys=[]; fixed → replays the subsequent event,
  mapping key _GET_stream, stream stays open >500 ms.

Regression protection is unit-level (examples/shared); the repo's
integration/e2e doubles use the same split('_')[0] pattern and never exercise
underscore stream IDs — intentionally left out of scope.

Known limitations (pre-existing, not introduced by this PR)

• Same-millisecond events can replay out of order or be dropped (eventId string
  sort includes the random suffix); measured locally, deserves its own issue.
• No changeset: @mcp-examples/shared is private: true and listed in
  .changeset/config.json ignore.

AI disclosure

This PR was developed with AI assistance (implementation, tests, and independent
reviews by separate AI reviewers under human supervision). All changes were
verified locally before submission (RED→GREEN tests, lint, typecheck, e2e).