fix: call onerror for silently swallowed transport errors

[qing-ant]

Summary

Fixes #1395 - Transport errors are silently swallowed due to missing onerror callback invocation.

Several handleRequest error paths in WebStandardStreamableHTTPServerTransport were returning error responses via createJsonErrorResponse() without calling the onerror callback, making transport errors invisible to consumers who set transport.onerror.

Changes

packages/server/src/server/streamableHttp.ts

Added this.onerror?.(new Error(...)) calls at 16 locations:

• handleGetRequest: Missing Accept header (406), concurrent SSE stream conflict (409)
• replayEvents: Event store not configured (400), invalid event ID format (400), stream conflict (409)
• handlePostRequest: Invalid Accept header (406), unsupported Content-Type (415), JSON parse error (400), invalid JSON-RPC message (400), re-initialization attempt (400), multiple init requests (400)
• validateSession: Server not initialized (400), missing session ID (400), session ID mismatch (404)
• validateProtocolVersion: Unsupported protocol version (400)
• handleUnsupportedRequest: Method not allowed (405)
• writeSSEEvent: SSE write failure (was silently returning false)

packages/server/test/server/streamableHttp.test.ts

Added 11 test cases verifying onerror is called for each error condition:

• Invalid JSON
• Invalid JSON-RPC message
• Missing Accept header on POST
• Unsupported Content-Type
• Server not initialized
• Invalid session ID
• Re-initialization attempt
• Missing Accept header on GET
• Concurrent SSE streams
• Unsupported protocol version
• Unsupported HTTP method

Pattern Used

Follows the existing pattern already present in the codebase (e.g., validateRequestHeaders):

this.onerror?.(new Error('Error message here'));
return this.createJsonErrorResponse(status, code, 'Error message here');

Testing

• pnpm typecheck:all passes
• pnpm --filter @modelcontextprotocol/server test — all tests pass (existing + 11 new)
• pnpm build:all succeeds

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Latest commit: 7e10f2c

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commit: 7e10f2c