test: autouse gh-106749 heal fixture so CTracer desync cannot cross tests

Under xdist, a desync at the end of one test carried into the start of
the next on the same worker; the missed lines moved with worker test
ordering (last seen as tests/shared/test_streamable_http.py:1399-1404,
previously :2072-2079). The per-throw-site heals in src/ covered every
SDK site we found; this backstops any remaining ones in test code or
test deps.

The heal line itself runs while the tracer is desynced (and is dead on
non-3.11), hence lax no cover.

Author: maxisbey

tests/conftest.py

@@ -1,6 +1,8 @@
 import os
-from collections.abc import Iterator
+import sys
+from collections.abc import AsyncIterator, Iterator
 
+import anyio.lowlevel
 import pytest
 
 # OpenTelemetry's `set_tracer_provider` is set-once per process, so the suite
@@ -23,6 +25,31 @@ def anyio_backend():
     return "asyncio"
 
 
+@pytest.fixture(autouse=True)
+async def _heal_gh106749(anyio_backend: str) -> AsyncIterator[None]:
+    """Re-sync coverage's CTracer after every async test on CPython 3.11.
+
+    CPython gh-106749: anyio delivers a task-group cancel via `coro.throw()`,
+    which on 3.11 skips the outer await chain's `'call'` trace events.
+    coverage.py's CTracer keys its frame stack on those events, so until the
+    next `.send()` resumption, line events are misattributed and dropped. Under
+    xdist a desync at the end of one test carries into the start of the next on
+    the same worker; the missed lines move with worker test ordering.
+
+    `cancel_shielded_checkpoint()` resumes via `.send()`, re-stamping the
+    missing events. The shielded variant is a pure scheduling yield with no
+    cancel delivery, so it cannot perturb a test's cancel-scope nesting. The
+    `anyio_backend` dependency makes anyio's pytest plugin own this fixture so
+    it runs in the test's task; sync tests skip it.
+    """
+    yield
+    # The heal line itself runs while the tracer is desynced (that's the
+    # point), so it cannot record its own execution; on non-3.11 the body is
+    # never entered. Hence lax no cover on the whole tail.
+    if sys.version_info[:2] == (3, 11):  # pragma: lax no cover
+        await anyio.lowlevel.cancel_shielded_checkpoint()
+
+
 @pytest.fixture(name="capfire")
 def _capfire_isolated(capfire: CaptureLogfire) -> Iterator[CaptureLogfire]:
     """Override of logfire's `capfire` that scopes the MCP tracer to the test.