The `_update_sample_rate` method modifies `baggage.sentry_items["sample_rate"]` without checking if the baggage is mutable. According to the `Baggage` class documentation (lines 608-611 in tracing_utils.py), callers must verify `baggage.mutable` is `True` before mutating the object. When a baggage is parsed from an incoming header that already contains sentry items, `mutable` is set to `False`. This means in continued traces, the sample rate update would modify an immutable baggage, violating the documented invariant.

The `NoOpStreamedSpan` class has been extended with sampling-related functionality (`unsampled_reason` parameter, client report recording in `_end()`), but there are no tests covering this new behavior. The existing `tests/tracing/test_noop_span.py` tests `NoOpSpan` but not `NoOpStreamedSpan`. Per the review checklist, every PR should have appropriate test coverage for functional changes.