test(mcp): stay connected until the SSE reply ends; drop cross-context reset

The test client reported http.disconnect as soon as the request body was
read, and the SDK's SSE response cancels its stream on disconnect, so
tools/list came back 200 with an empty body. Model a real client: wait
for the response (with a 15s deadline), then disconnect.

The caller fixture reset its ContextVar from sync teardown, a different
context from the async test that set it; each async test already runs in
its own context copy, so the reset is dropped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/mcp/test_server.py

@@ -26,13 +26,12 @@ OPERATOR = Caller(user_id="u1", username="ana", role=UserRole.operator, token_id
 
 @pytest.fixture
 def as_caller():
-    tokens = []
-
+    # Each async test runs in its own task, i.e. a copy of the context, so a
+    # caller set here can't leak into another test; resetting from the sync
+    # teardown would fail ("created in a different Context").
     def _set(caller):
-        tokens.append(_context._caller_ctx.set(caller))
-    yield _set
-    for t in reversed(tokens):
-        _context._caller_ctx.reset(t)
+        _context._caller_ctx.set(caller)
+    return _set
 
 
 def _server(plugins_cfg=None):