fix(server): deregister terminated streamable HTTP sessions

[mikemikimike]

Summary

• remove terminated existing sessions from both session registries immediately after handling the request
• add a regression test for the clean DELETE/session termination path

Fixes #3300

Validation

• uv run --frozen ruff format ...
• uv run --frozen ruff check src/mcp/server/streamable_http_manager.py tests/server/test_streamable_http_manager.py
• uv run --frozen pyright src/mcp/server/streamable_http_manager.py tests/server/test_streamable_http_manager.py
• uv run --frozen pytest tests/server/test_streamable_http_manager.py -q (31 passed)
• git diff --check

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="tests/server/test_streamable_http_manager.py">

<violation number="1" location="tests/server/test_streamable_http_manager.py:316">
P3: This regression test does not verify `_session_owners` cleanup because it never adds `session_id` to `_session_owners` before the request. Seed that registry first so the assertion can fail if owner removal breaks.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P3: This regression test does not verify _session_owners cleanup because it never adds session_id to _session_owners before the request. Seed that registry first so the assertion can fail if owner removal breaks.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At tests/server/test_streamable_http_manager.py, line 316:

<comment>This regression test does not verify `_session_owners` cleanup because it never adds `session_id` to `_session_owners` before the request. Seed that registry first so the assertion can fail if owner removal breaks.</comment>

<file context>
@@ -304,6 +304,36 @@ async def mock_receive():
+    transport = AsyncMock()
+    transport.is_terminated = True
+    transport.idle_scope = None
+    manager._server_instances[session_id] = transport
+
+    scope = {
</file context>