feat(httpx): Migrate to span first

[ericapisani]

Migrates the httpx integration to the span-first (streaming span) architecture.

When _experiments={"trace_lifecycle": "stream"} is enabled, the integration
now uses StreamedSpan via sentry_sdk.traces.start_span instead of the legacy
Span-based (transactions) path. The legacy path remains unchanged for backwards compatibility.

Supporting changes:

• add_http_request_source_for_streamed_span() added to tracing_utils — a
  StreamedSpan-compatible variant that must be called inside the span context
  manager so source data is attached before the span is flushed
• add_source() updated to dispatch between set_data/set_attribute depending
  on span type
• start_timestamp_monotonic_ns property added to StreamedSpan and
  NoOpStreamedSpan to support the request source duration threshold check

Part of the broader span-first integration migration.

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Semver Impact of This PR

🟡 Minor (new features)

📋 Changelog Preview

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New Features ✨

• (ci) Cancel in-progress PR workflows on new commit push by joshuarli in #5994

• (httpx) Migrate to span first by ericapisani in #6084

• Add db.driver.name spans to database integrations by ericapisani in #6082

Bug Fixes 🐛

• (google_genai) Redact binary data in inline_data and fix multi-part message extraction by ericapisani in #5977
• (grpc) Add isolation_scope to async server interceptor by robinvd in #5940
• (profiler) Stop nulling buffer on teardown by ericapisani in #6075

Internal Changes 🔧

• (celery) Remove unused NoOpMgr from utils by sentrivana in #6078
• (pydantic-ai) Remove dead Model.request patch by alexander-alderman-webb in #5956
• (tests) Replace deprecated enable_tracingwith traces_sample_rate by sentrivana in #6077
• Set explicit base-branch for codecov action by ericapisani in #5992

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Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 12.80s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

❌ Patch coverage is 7.56%. Project has 15103 uncovered lines.
❌ Project coverage is 29.55%. Comparing base (base) to head (head).

Files with missing lines (3)

File	Patch %	Lines
tracing_utils.py	36.64%	⚠️ 486 Missing and 28 partials
traces.py	36.77%	⚠️ 196 Missing
httpx.py	11.21%	⚠️ 103 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
- Coverage    29.61%    29.55%    -0.06%
==========================================
  Files          190       190         —
  Lines        21365     21437       +72
  Branches      7068      7108       +40
==========================================
+ Hits          6327      6334        +7
- Misses       15038     15103       +65
- Partials       477       477         —

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Codecov Results 📊

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