fix(server): prevent SSE disconnect from crashing inspector proxy

[syf2211]

Summary

Prevent the inspector proxy from crashing with an unhandled Not connected error when an SSE client disconnects or is replaced (e.g. browser refresh, multiple tabs, or rapid reconnects).

Motivation

Fixes #1014. When the browser closes an SSE session, SSEServerTransport.send() throws if the stream is already torn down. The stdio stderr forwarder and mcpProxy could still call send() on in-flight server messages, producing an unhandled rejection that crashes the inspector process.

Unlike Streamable HTTP, the legacy /stdio and /sse routes also never removed stale entries from the proxy session maps on client disconnect.

Changes

• Add sessionRegistry helpers:
  • sendToClientSafe() — catches disconnected SSE sends instead of crashing
  • removeSession() — clears proxy session maps
  • chainOnClose() — runs cleanup before existing mcpProxy teardown
• /stdio: use safe sends for stderr notifications; remove stderr listener and session maps on client close
• /sse: remove session maps on client close
• mcpProxy: route client-bound sends through sendToClientSafe
• Add unit tests for the new helpers

Tests

• cd server && npm run build — pass
• cd server && npm test — 3/3 pass
• npm run prettier-check — pass

Notes

• The fix is intentionally minimal and scoped to V1 proxy session lifecycle.
• Error matching uses the SDK's "Not connected" message, consistent with other MCP transports.

[cliffhall]

Closing: v1 is deprecated.

Thank you for this contribution, and apologies for the long wait for a response.

v1 will receive security fixes only. We reviewed every open v1 PR for security impact before closing — see the backlog triage in #1819 — and a small number were retained for a final 1.0.5 patch release. This one is a functionality, compatibility, or cleanup change rather than a vulnerability fix, so it is being closed unmerged. This is not a judgment on the quality of the work — it's a consequence of the branch it targets being frozen.

If the underlying problem still exists in v2, we'd genuinely like to know. Please open an issue describing it against v2. Note that we accept external contributions as issues rather than pull requests — maintainers handle design and implementation through a prompt-driven workflow. See CONTRIBUTORS.md.

Thanks again for taking the time to contribute to the Inspector.