fix(streamable_http): expose session_idle_timeout and don't reap active sessions

[BuildWithAbid]

Closes #2455

Fixes both problems described in the issue:

Problem 1: session_idle_timeout not exposed by streamable_http_app()

StreamableHTTPSessionManager(...) accepted session_idle_timeout, but neither Server.streamable_http_app(...) nor MCPServer.streamable_http_app(...) did. So the canonical high-level API could not configure a supported session-manager feature.

This PR threads session_idle_timeout: float | None = None through both wrappers and forwards it to the underlying StreamableHTTPSessionManager.

Problem 2: active requests cancelled by the idle timeout

The previous logic reset the deadline only at the start of each incoming request. A handler whose work outlived session_idle_timeout would be cancelled mid-flight; the client waited until session termination instead of receiving the tool result.

Replaced the deadline-bumping logic with a per-transport in-flight counter and a _suspend_idle_timeout() async context manager:

• on entry: idle_active_requests += 1. If this is the first concurrent request, set idle_scope.deadline = math.inf.
• on exit: idle_active_requests -= 1. If this was the last in-flight request, restore now + session_idle_timeout.

Both the new-session and existing-session code paths now wrap transport.handle_request(...) with _suspend_idle_timeout, so the deadline is suspended for the lifetime of every request.

Test plan

All in tests/server/test_streamable_http_manager.py:

• test_session_idle_timeout_passthrough_lowlevel — verifies the kwarg propagates from Server.streamable_http_app() to the manager.
• test_session_idle_timeout_passthrough_mcpserver — same for MCPServer.streamable_http_app().
• test_suspend_idle_timeout_sets_deadline_inf_then_restores — direct unit test of the helper: deadline is inf during the request, finite (≈ now + timeout) after.
• test_suspend_idle_timeout_only_restores_after_last_concurrent_request — nested suspensions keep the deadline at inf until the outermost exit; counter logic correctness.
• test_suspend_idle_timeout_no_op_without_timeout — helper is inert when no timeout is configured.
• test_long_running_request_outlives_idle_timeout — regression for problem 2: end-to-end via httpx.ASGITransport + Client, a tool whose handler sleeps 4× the timeout still completes successfully.
• test_idle_session_reaped_after_request_completes — the session is still reaped once the request finishes, so existing reaping behavior is preserved.

Local results:

```
tests/server/test_streamable_http_manager.py 18 passed
tests/server/ + tests/shared/test_streamable_http.py 542 passed
ruff check + ruff format clean
pyright 0 errors, 0 warnings
```

Notes

• No public API removed — session_idle_timeout is a new kwarg with the same None default everywhere.
• The validation guards (session_idle_timeout > 0, mutually exclusive with stateless) still live on StreamableHTTPSessionManager.__init__ and continue to raise correctly when invoked through either wrapper.