Streamable proxy drops/fails requests under high concurrent load (fixed 100-buffer channels, no backpressure)

Summary

The streamable HTTP→stdio proxy forwards through two fixed-size (buffered=100) channels with non-blocking, drop-on-full sends, drained by single-consumer goroutines. Under high concurrent in-flight load (>~100 simultaneous requests) it degrades ungracefully — hard-failing or dropping — rather than applying backpressure. This is especially relevant to the Modern (2026-07-28) stateless path, which invites high concurrency (no session pinning).

Detail

pkg/transport/proxy/streamable/streamable_proxy.go:

• messageCh/responseCh are buffered at 100 (hardcoded ~L175-176; no Option to configure).
• SendMessageToDestination (~L295) and ForwardResponseToClients (~L305): select { case ch <- msg: return nil; default: return err } — non-blocking; on a full channel they fail/drop immediately.
• dispatchResponses drains responseCh in a single goroutine.

If either consumer is briefly descheduled while >100 requests are in flight:

• messageCh full → SendMessageToDestination errors → doRequest errors → HTTP 500.
• responseCh full → response dropped → the waiting client blocks until requestTimeout (60s default) → HTTP 504.

Evidence

Observed while building the Modern no-accumulation resilience test (#5837): a 1000-request concurrent burst intermittently produced a 504, root-caused by an MoE review to this backpressure gap (worse under -race scheduling). The test now throttles its own concurrency to sidestep it, but the production behavior remains.

Suggested direction

Make the buffer sizes configurable and/or apply real backpressure (blocking or context-bounded send) instead of drop-on-full, so burst load degrades in latency rather than dropped/failed requests. At minimum, document the ~100 concurrent-in-flight ceiling.

Context

Enhancement / resilience, surfaced by #5837 Tier-3 testing. Not a regression; pre-existing design limitation.

Read pkg/transport/proxy/streamable/streamable_proxy.go, focusing on the messageCh and responseCh setup, SendMessageToDestination, ForwardResponseToClients, and dispatchResponses. Reproduce the burst behavior described from the Modern resilience test in #5837, then verify that concurrent requests no longer fail or time out because either channel is full while preserving request cancellation and timeout behavior.