integration: add stress and multiplexing tests

[shubhamdhama]

Add stress tests targeting concurrency bugs in the multiplexing stack
(MuxWriter, packetQueue, manager, activeStreams) and correctness tests
for stream isolation under cancel, error, and connection close.

Stress tests (stress_test.go):

• SustainedConcurrentStreams: 50 bidi streams x 100 echo messages,
  validates no cross-stream data corruption.
• RapidOpenCloseCycles: 500 sequential create/use/destroy cycles on one
  connection.
• CancelStorm: 30 streams with ~50% randomly cancelled mid-traffic,
  verifying cancel isolation.
• ShutdownDuringActivity: conn.Close() with 20 active streams, deadlock
  detection.
• MixedRPCTypes: 30 goroutines x 10 rounds of randomly chosen RPC types
  (unary, client-streaming, server-streaming, bidi) on one connection.
• ConcurrentCancelCloseTransportClose: triple-race shutdown (cancel +
  conn.Close + transport.Close) with active streams.
• ConcurrentUnary: 100 goroutines x 50 unary RPCs (sustained throughput).
• BurstUnary: 1000 goroutines x 1 unary RPC (burst contention).

Multiplexing tests (multiplex_test.go):

• CancelIsolation, ErrorIsolation, ConnCloseWithActiveStreams,
  TransportCloseTerminatesAllStreams.

Transport selection is randomized per test (pipe or TCP loopback), with
a -stress.transport flag override for deterministic reproduction. All
tests use goleak for goroutine leak detection.

Moves TestConcurrentStreams and TestConcurrent from simple_test.go into
the stress suite as SustainedConcurrentStreams and BurstUnary.