Run StreamableHTTP transport tests in process instead of over sockets

Final installment of the in-process test migration: this was the last
file spawning uvicorn subprocesses on bind-then-close ports with
readiness polling, which races under pytest-xdist when two workers pick
the same ephemeral port. Two tests in this file have flaked exactly
that way under parallel load.

All four subprocess servers (basic, JSON-response, event-store,
context-aware) become in-process apps served through the interaction
suite's StreamingASGITransport, held open by the session manager's
run() context. Raw `requests` calls become httpx calls against the
bridge client; the sync request-validation tests become anyio tests.
The second-GET-409 test now holds the first stream open by
construction, where the subprocess version noted it "might fail if the
first stream fully closed before this runs".

Assertions are unchanged, with documented exceptions now that the
server handlers run as traced in-process code:

- The long_running_with_checkpoints tool and the slow:// resource
  branch had no callers and are removed, so the expected tools/list
  count drops from 10 to 9 in five tests.
- Dead defensive arms become asserts (sampling non-text fallback,
  close_sse_stream truthiness checks, the context server's unknown-tool
  fallthrough and request checks), and the event store's
  replay-from-unknown-event arm becomes a lookup that requires a stored
  event, since unreachable branches now fail branch coverage instead of
  hiding in an untraced subprocess.
- test_client_crash_handled no longer sleeps between crashing clients;
  the bridge drains each client's teardown before the next connects.

Three pragmas in src/mcp/server/streamable_http.py covered only by the
formerly untraced subprocess (close_standalone_sse_stream, its session
message callback, and the JSON-mode Accept rejection) are now executed
by traced tests and removed.

With the last wait_for_server user migrated, the helper is deleted from
tests/test_helpers.py; run_uvicorn_in_thread stays for the websocket
smoke test.

Author: maxisbey

src/mcp/server/streamable_http.py

@@ -207,7 +207,7 @@ def close_sse_stream(self, request_id: RequestId) -> None:
             send_stream.close()
             receive_stream.close()
 
-    def close_standalone_sse_stream(self) -> None:  # pragma: no cover
+    def close_standalone_sse_stream(self) -> None:
         """Close the standalone GET SSE stream, triggering client reconnection.
 
         This method closes the HTTP connection for the standalone GET stream used
@@ -221,8 +221,6 @@ def close_standalone_sse_stream(self) -> None:  # pragma: no cover
             This is a no-op if there is no active standalone SSE stream.
             Requires event_store to be configured for events to be stored during
             the disconnect.
-            Currently, client reconnection for standalone GET streams is NOT
-            implemented - this is a known gap (see test_standalone_get_stream_reconnection).
         """
         self.close_sse_stream(GET_STREAM_KEY)
 
@@ -245,7 +243,7 @@ def _create_session_message(
             async def close_stream_callback() -> None:
                 self.close_sse_stream(request_id)
 
-            async def close_standalone_stream_callback() -> None:  # pragma: no cover
+            async def close_standalone_stream_callback() -> None:
                 self.close_standalone_sse_stream()
 
             metadata = ServerMessageMetadata(
@@ -421,7 +419,7 @@ async def _validate_accept_header(self, request: Request, scope: Scope, send: Se
         has_json, has_sse = self._check_accept_headers(request)
         if self.is_json_response_enabled:
             # For JSON-only responses, only require application/json
-            if not has_json:  # pragma: no cover
+            if not has_json:
                 response = self._create_error_response(
                     "Not Acceptable: Client must accept application/json",
                     HTTPStatus.NOT_ACCEPTABLE,