fix(flask): Set user data on scope at request start

[ericapisani]

Previously, Flask user data (id, email, username) was only set via the _add_user_to_event event processor, which runs at event capture time. Under span streaming, spans are sent before an error event is captured, so user attributes were missing from those spans.

This refactors the user property logic into a shared _get_flask_user_properties() helper and calls it in _request_started to set user data directly on the isolation scope. The event processor continues to use the same helper to keep behavior consistent for non-streaming cases. Tests are extended to cover both span streaming and non-streaming paths.

Fixes GH-6565
Refs PY-2528

[linear-code]

PY-2528

[github-actions]

Codecov Results 📊

✅ 90696 passed | ⏭️ 6031 skipped | Total: 96727 | Pass Rate: 93.76% | Execution Time: 310m 45s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +132
Passed Tests	📈 +132
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 90.63%. Project has 2421 uncovered lines.
❌ Project coverage is 89.75%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
sentry_sdk/integrations/flask.py	90.63%	⚠️ 3 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
- Coverage    89.77%    89.75%    -0.02%
==========================================
  Files          192       192         —
  Lines        23615     23630       +15
  Branches      8130      8140       +10
==========================================
+ Hits         21201     21209        +8
- Misses        2414      2421        +7
- Partials      1340      1340         —

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