feat: add idle timeout for StreamableHTTP sessions (#1994)

CLAUDE.md

@@ -24,6 +24,11 @@ This document contains critical information about working with this codebase. Fo
    - Coverage: test edge cases and errors
    - New features require tests
    - Bug fixes require regression tests
+   - Avoid `anyio.sleep()` with a fixed duration to wait for async operations. Instead:
+     - Use `anyio.Event` — set it in the callback/handler, `await event.wait()` in the test
+     - For stream messages, use `await stream.receive()` instead of `sleep()` + `receive_nowait()`
+     - Exception: `sleep()` is appropriate when testing time-based features (e.g., timeouts)
+   - Wrap indefinite waits (`event.wait()`, `stream.receive()`) in `anyio.fail_after(5)` to prevent hangs
 
 - For commits fixing bugs or adding features based on user reports add: