fix(client): wait for in-flight requests before aborting on close()

[aptsalt]

Summary

• Track in-flight send() requests in both SSEClientTransport and StreamableHTTPClientTransport
• close() now waits for pending requests to settle (with a 2s timeout) before calling AbortController.abort()
• Prevents Undici/OpenTelemetry from marking successful 200 OK responses as UND_ERR_ABORTED

Context

Fixes #1231 — When close() is called while POST requests are still in-flight, the immediate AbortController.abort() causes Undici to report UND_ERR_ABORTED even though the server returned 200 OK. This creates misleading telemetry in OpenTelemetry traces.

Changes

packages/client/src/client/streamableHttp.ts

• Added _pendingRequests Set to track active send() calls
• send() registers/unregisters itself from the set using try/finally
• close() waits up to 2s for pending requests before aborting

packages/client/src/client/sse.ts

• Same pattern applied to the SSE transport

packages/client/test/client/streamableHttp.test.ts

• "should wait for in-flight requests before aborting" — verifies abort fires after the fetch completes
• "should abort after timeout even if requests are still pending" — verifies stuck requests don't block close() beyond 2s

Test plan

• All 40 StreamableHTTP tests pass
• All 27 SSE tests pass
• Build succeeds (pnpm build:all)
• Manual verification with OpenTelemetry Undici instrumentation

Closes #1231

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🦋 Changeset detected

Latest commit: d215fff

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

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/client	Patch

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