ref(tracing): No-op and emit warning in start_transaction with the streaming trace lifecycle

[alexander-alderman-webb]

Description

Mirror the deprecation warning in Scope.start_span(). The API is not available when the streaming trace lifecycle is used.

Adapt tests that wrongly assert on transactions when the lifecycle is enabled.

Issues

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 style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[github-actions]

Codecov Results 📊

✅ 91914 passed | ⏭️ 6302 skipped | Total: 98216 | Pass Rate: 93.58% | Execution Time: 311m 19s

📊 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 100.00%. Project has 2478 uncovered lines.
✅ Project coverage is 89.68%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.68%    89.68%        —%
==========================================
  Files          193       193         —
  Lines        24007     24010        +3
  Branches      8342      8344        +2
==========================================
+ Hits         21530     21532        +2
- Misses        2477      2478        +1
- Partials      1386      1387        +1

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