Stabilize Python streaming resume E2E test (#2597)

* Stabilize Python streaming resume E2E test

Wait for the detached session lock to release before cold-resuming it from a new CLI process, avoiding a timing race in merge-queue runs.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

* Cover both Python streaming resume paths

Reuse the session-lock release wait before every cold-resume path in the streaming fidelity E2E test.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: roji

python/e2e/test_streaming_fidelity_e2e.py

@@ -5,12 +5,29 @@
 import pytest
 
 from copilot import CopilotClient, RuntimeConnection
+from copilot.rpc import SessionsCheckInUseRequest
 from copilot.session import PermissionHandler
 
-from .testharness import E2ETestContext
+from .testharness import E2ETestContext, wait_for_condition
 
 pytestmark = pytest.mark.asyncio(loop_scope="module")
 
+RESUME_LOCK_TIMEOUT = 60.0
+
+
+async def _wait_for_session_lock_release(ctx: E2ETestContext, session_id: str) -> None:
+    async def session_lock_is_released() -> bool:
+        result = await ctx.client.rpc.sessions.check_in_use(
+            SessionsCheckInUseRequest(session_ids=[session_id])
+        )
+        return session_id not in result.in_use
+
+    await wait_for_condition(
+        session_lock_is_released,
+        timeout=RESUME_LOCK_TIMEOUT,
+        timeout_message=f"Timed out waiting for session '{session_id}' to release its lock.",
+    )
+
 
 class TestStreamingFidelity:
     async def test_should_produce_delta_events_when_streaming_is_enabled(self, ctx: E2ETestContext):
@@ -70,9 +87,12 @@ async def test_should_produce_deltas_after_session_resume(self, ctx: E2ETestCont
         session = await ctx.client.create_session(
             on_permission_request=PermissionHandler.approve_all, streaming=False
         )
+        session_id = session.session_id
         await session.send_and_wait("What is 3 + 6?")
         await session.disconnect()
 
+        await _wait_for_session_lock_release(ctx, session_id)
+
         # Resume using a new client
         github_token = (
             "fake-token-for-e2e-tests" if os.environ.get("GITHUB_ACTIONS") == "true" else None
@@ -86,7 +106,7 @@ async def test_should_produce_deltas_after_session_resume(self, ctx: E2ETestCont
 
         try:
             session2 = await new_client.resume_session(
-                session.session_id,
+                session_id,
                 on_permission_request=PermissionHandler.approve_all,
                 streaming=True,
             )
@@ -126,6 +146,8 @@ async def test_should_not_produce_deltas_after_session_resume_with_streaming_dis
         session_id = session.session_id
         await session.disconnect()
 
+        await _wait_for_session_lock_release(ctx, session_id)
+
         # Resume with streaming disabled
         new_client = CopilotClient(
             connection=RuntimeConnection.for_stdio(path=ctx.cli_path),