Fix resource update crash after client disconnect

[stephenreid321]

Summary

• Skip resource update notifications once the MCP server transport is disconnected.
• Catch/log resource update notification failures so a late subscription update cannot become an unhandled rejection.
• Add unit coverage for disconnected and late-failing resource update paths.

Context

When a Streamable HTTP client subscribes to debug://mongodb, lets the session go idle, and the session closes, reactive resources can still emit sendResourceUpdated. In that state the MCP SDK throws Error: Not connected; because the promise was intentionally discarded, the rejection can terminate Node and restart hosted services.

Observed stack shape:

Error: Not connected
    at Server.notification (.../@modelcontextprotocol/sdk/dist/esm/shared/protocol.js:791:19)
    at Server.sendResourceUpdated (.../@modelcontextprotocol/sdk/dist/esm/server/index.js:423:21)
    at Server.sendResourceUpdated (.../mongodb-mcp-server/dist/esm/server.js:121:40)
    at DebugResource.triggerUpdate (.../mongodb-mcp-server/dist/esm/resources/resource.js:38:26)

Validation

• npx --yes prettier@3.8.1 --check src/server.ts tests/unit/server.test.ts
• Secret scan of changed snippets: no secrets found

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