## Summary
- `preserve_contexts_async_generator` left `PROVIDER_DATA_VAR` (and
other context vars) populated after a streaming generator completed on
HEAD~1, so the asyncio context for request N+1 started with request N's
provider payload.
- FastAPI dependencies and middleware execute before
`request_provider_data_context` rebinds the header data, meaning
auth/logging hooks could observe a prior tenant's credentials or treat
them as authenticated. Traces and any background work that inspects the
context outside the `with` block leak as well—this is a real security
regression, not just a CLI artifact.
- The wrapper now restores each tracked `ContextVar` to the value it
held before the iteration (falling back to clearing when necessary)
after every yield and when the generator terminates, so provider data is
wiped while callers that set their own defaults keep them.
## Test Plan
- `uv run pytest tests/unit/core/test_provider_data_context.py -q`
- `uv run pytest tests/unit/distribution/test_context.py -q`
Both suites fail on HEAD~1 and pass with this change.
