test: Add streaming sample_rand propagation tests (#6780)

### Description
<!-- What changed and why? -->

#### 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)
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style (`feat:`, `fix:`, `ref:`, `meta:`)
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Author: sentrivana

tests/tracing/test_sample_rand_propagation.py

@@ -25,6 +25,22 @@ def test_continue_trace_with_sample_rand():
     assert transaction.get_baggage().sentry_items["sample_rand"] == "0.1"
 
 
+def test_continue_trace_with_sample_rand_span_streaming(sentry_init):
+    """
+    Test that an incoming sample_rand is propagated onto the segment's baggage.
+    """
+    sentry_init(traces_sample_rate=1.0, _experiments={"trace_lifecycle": "stream"})
+
+    headers = {
+        "sentry-trace": "00000000000000000000000000000000-0000000000000000-1",
+        "baggage": "sentry-sample_rand=0.100000,sentry-sample_rate=0.5",
+    }
+
+    sentry_sdk.traces.continue_trace(headers)
+    with sentry_sdk.traces.start_span(name="span") as segment:
+        assert segment._get_baggage().sentry_items["sample_rand"] == "0.100000"
+
+
 def test_continue_trace_missing_sample_rand():
     """
     Test that a missing sample_rand is filled in onto the transaction's baggage.
@@ -41,3 +57,23 @@ def test_continue_trace_missing_sample_rand():
         transaction = sentry_sdk.continue_trace(headers)
 
     assert transaction.get_baggage().sentry_items["sample_rand"] == "0.500000"
+
+
+def test_continue_trace_missing_sample_rand_span_streaming(sentry_init):
+    """
+    Test that a missing sample_rand is filled in onto the segment's baggage.
+    """
+
+    sentry_init(traces_sample_rate=1.0, _experiments={"trace_lifecycle": "stream"})
+
+    headers = {
+        "sentry-trace": "00000000000000000000000000000000-0000000000000000",
+        "baggage": "sentry-placeholder=asdf",
+    }
+
+    with mock.patch(
+        "sentry_sdk.tracing_utils.Random.randrange", Mock(return_value=500000)
+    ):
+        sentry_sdk.traces.continue_trace(headers)
+        with sentry_sdk.traces.start_span(name="span") as span:
+            assert span._get_baggage().sentry_items["sample_rand"] == "0.500000"