fix: resume in-memory event streams with underscored ids

[Genmin]

Summary

• stop deriving the in-memory event store stream id by splitting event ids on underscores
• look up the stored stream id directly so the standalone SSE stream id _GET_stream can resume correctly
• add a regression test that stores two _GET_stream events and verifies replay after the first event returns the original stream id and sends the second event

Fixes #943.

Validation

• pnpm --filter @modelcontextprotocol/server test -- test/integration/test/taskResumability.test.ts -t "standalone SSE stream IDs"
• pnpm --filter @modelcontextprotocol/test-integration test -- test/taskResumability.test.ts -t "standalone SSE stream IDs"
• pnpm --filter @modelcontextprotocol/examples-server typecheck
• pnpm --filter @modelcontextprotocol/examples-server lint
• pnpm --filter @modelcontextprotocol/test-integration lint
• pre-push: full typecheck:all, build:all, and lint:all

[changeset-bot]

🦋 Changeset detected

Latest commit: 991ad4a

The changes in this PR will be included in the next version bump.

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commit: 991ad4a

[Genmin]

Added the missing patch changeset for the touched example/integration packages. The previous red test was isolated to the unrelated Miniflare Cloudflare Worker Network connection lost failure, so this should at least clear the release-metadata blocker and trigger a fresh run.