test: Add sample_rand span streaming tests (#6781)

### Description
Add streaming counterparts of the tests in `test_sample_rand.py`.

#### Issues
Part of #5395

#### Reminders
- Please add tests to validate your changes, and lint your code using
`uv run ruff`.
- Add GH Issue ID _&_ Linear ID (if applicable)
- PR title should use [conventional
commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

Author: sentrivana

tests/tracing/test_sample_rand.py

@@ -32,6 +32,38 @@ def test_deterministic_sampled(sentry_init, capture_events, sample_rate, sample_
     assert len(events) == int(sample_rand < sample_rate)
 
 
+@pytest.mark.parametrize("sample_rand", (0.0, 0.25, 0.5, 0.75))
+@pytest.mark.parametrize("sample_rate", (0.0, 0.25, 0.5, 0.75, 1.0))
+def test_deterministic_sampled_span_streaming(
+    sentry_init, capture_items, sample_rate, sample_rand
+):
+    """
+    Test that sample_rand is generated on new traces, that it is used to
+    make the sampling decision, and that it is included in the segment's
+    baggage.
+    """
+    sentry_init(
+        traces_sample_rate=sample_rate,
+        trace_lifecycle="stream",
+    )
+    items = capture_items("span")
+
+    with mock.patch(
+        "sentry_sdk.tracing_utils.Random.randrange",
+        return_value=int(sample_rand * 1000000),
+    ):
+        with sentry_sdk.traces.start_span(name="span") as span:
+            assert (
+                span._get_baggage().sentry_items["sample_rand"] == f"{sample_rand:.6f}"  # noqa: E231
+            )
+
+    sentry_sdk.flush()
+
+    # Span captured if sample_rand < sample_rate, indicating that
+    # sample_rand is used to make the sampling decision.
+    assert len(items) == int(sample_rand < sample_rate)
+
+
 @pytest.mark.parametrize("sample_rand", (0.0, 0.25, 0.5, 0.75))
 @pytest.mark.parametrize("sample_rate", (0.0, 0.25, 0.5, 0.75, 1.0))
 def test_transaction_uses_incoming_sample_rand(
@@ -54,3 +86,36 @@ def test_transaction_uses_incoming_sample_rand(
     # Transaction event captured if sample_rand < sample_rate, indicating that
     # sample_rand is used to make the sampling decision.
     assert len(events) == int(sample_rand < sample_rate)
+
+
+@pytest.mark.parametrize("sample_rand", (0.0, 0.25, 0.5, 0.75))
+@pytest.mark.parametrize("sample_rate", (0.0, 0.25, 0.5, 0.75, 1.0))
+def test_segment_uses_incoming_sample_rand_span_streaming(
+    sentry_init, capture_items, sample_rate, sample_rand
+):
+    """
+    Test that the segment uses the sample_rand value from the incoming baggage.
+    """
+    sentry_init(
+        traces_sample_rate=sample_rate,
+        trace_lifecycle="stream",
+    )
+    items = capture_items("span")
+
+    sentry_sdk.traces.continue_trace(
+        {
+            "sentry-trace": "0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331",
+            "baggage": f"sentry-sample_rand={sample_rand:.6f}",
+        }
+    )
+
+    with sentry_sdk.traces.start_span(name="span") as span:
+        assert (
+            span._get_baggage().sentry_items["sample_rand"] == f"{sample_rand:.6f}"  # noqa: E231
+        )
+
+    sentry_sdk.flush()
+
+    # Span captured if sample_rand < sample_rate, indicating that
+    # sample_rand is used to make the sampling decision.
+    assert len(items) == int(sample_rand < sample_rate)