feat(spans): Add flush_id to flushed segment messages

[untitaker]

Add a unique UUID (flush_id) to each message produced by FlushedSegment.to_messages(). The UUID is generated at call time, right before JSON serialization.

This allows us to see whether the duplicate segments we observe "come from" redis, or whether they are created post-serialization.

ref INC-2116
ref STREAM-881

[github-actions]

Backend Test Failures

Failures on 8f5f70c in this run:

tests/sentry/spans/consumers/process/test_consumer.py::test_basic[None] — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/spans/consumers/process/test_consumer.py:83: in test_basic
    assert orjson.loads(msg.value) == {
E   AssertionError: assert {'flush_id': ...d': 12, ...}]} == {'spans': [{'...d': 12, ...}]}
E     
E     Omitting 1 identical items, use -vv to show
E     Left contains 1 more item:
E     {'flush_id': '8f0f55dffba7434e84a8b1a12bdb8dee'}
E     
E     Full diff:
E       {
E     +     'flush_id': '8f0f55dffba7434e84a8b1a12bdb8dee',
E           'spans': [
E               {
E                   'attributes': {
E                       'sentry.segment.id': {
E                           'type': 'string',
E                           'value': 'aaaaaaaaaaaaaaaa',
E                       },
E                   },
E                   'end_timestamp': 1700000000.0,
E                   'is_segment': True,
E                   'project_id': 12,
E                   'span_id': 'aaaaaaaaaaaaaaaa',
E                   'start_timestamp': 1699999999.0,
E                   'trace_id': 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb',
E               },
E           ],
E       }

tests/sentry/spans/consumers/process/test_consumer.py::test_basic[2] — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/spans/consumers/process/test_consumer.py:83: in test_basic
    assert orjson.loads(msg.value) == {
E   AssertionError: assert {'flush_id': ...d': 12, ...}]} == {'spans': [{'...d': 12, ...}]}
E     
E     Omitting 1 identical items, use -vv to show
E     Left contains 1 more item:
E     {'flush_id': '7a5d094aebb44cab9741d69ac9393508'}
E     
E     Full diff:
E       {
E     +     'flush_id': '7a5d094aebb44cab9741d69ac9393508',
E           'spans': [
E               {
E                   'attributes': {
E                       'sentry.segment.id': {
E                           'type': 'string',
E                           'value': 'aaaaaaaaaaaaaaaa',
E                       },
E                   },
E                   'end_timestamp': 1700000000.0,
E                   'is_segment': True,
E                   'project_id': 12,
E                   'span_id': 'aaaaaaaaaaaaaaaa',
E                   'start_timestamp': 1699999999.0,
E                   'trace_id': 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb',
E               },
E           ],
E       }

[linear-code]

STREAM-881 Investigate duplicate spans appearing after buffered-segments