fix(mcp): recover expired HTTP sessions

[mattzcarey]

Summary

• detect when a streamable HTTP session restored from storage is rejected by the server with HTTP 404
• clear the stale session id from memory and storage before performing a fresh initialize handshake
• rediscover once on the new session, while leaving ordinary discovery failures unchanged

Why

The MCP SDK skips initialization when given a persisted session id. If the server expired that session while the Durable Object slept, restore appeared connected but every discovery request returned 404. Because the id remained persisted, the same unusable session came back on every wake.

Recovery is deliberately narrow: it only handles the SDK's StreamableHTTPError with status 404 while probing a restored session. A JSON-RPC application error with code 404, or an HTTP 404 on a session negotiated in the current lifetime, does not trigger a reconnect.

Review follow-up

• narrowed the detector from any numeric code: 404 to the SDK's actual StreamableHTTPError, avoiding collisions with JSON-RPC application errors
• added a regression test proving the persisted id is cleared before reconnect, including when that fresh reconnect fails
• no verifier findings remain

Validation

• pnpm --filter agents exec vitest run src/tests/mcp — 30 files, 536 tests passed
• pnpm run check — passed
• integration coverage verifies stale-session rejection, fresh initialization, rediscovery, storage replacement, failed-reconnect cleanup, and the non-recovery guards

Includes a patch changeset for agents.

---

[changeset-bot]

🦋 Changeset detected

Latest commit: 2ff3881

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

This PR includes changesets to release 2 packages

Name	Type
agents	Patch
@cloudflare/agent-think	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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commit: 2ff3881