fix(dedupe): Track seen exceptions via attribute instead of ContextVar

[ericapisani]

Replace DedupeIntegration's weakref/ContextVar-based "last seen"
tracking with a _handled_by_sentry flag set directly on the
exception. This avoids the contextvar leaking across async contexts
and removes the need for reset_last_seen(), which the client used
to call after before_send dropped an exception.

Add a regression test for the case where the same exception instance
is re-raised (e.g. Django re-raising in middleware after a view
already handled it) to confirm it's still deduped correctly.

Fixes PY-2381
Fixes #6094

[linear-code]

PY-2381

[github-actions]

Codecov Results 📊

✅ 56706 passed | ⏭️ 2727 skipped | Total: 59433 | Pass Rate: 95.41% | Execution Time: 156m 7s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -80918
Passed Tests	📉 -76423
Failed Tests	—
Skipped Tests	📉 -4495

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2075 uncovered lines.
❌ Project coverage is 90%. Comparing base (185fb9c) to head (63e1e48).

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.35%    90.00%    -0.35%
==========================================
  Files          202       185       -17
  Lines        26551     20741     -5810
  Branches      9856      7158     -2698
==========================================
+ Hits         23988     18666     -5322
- Misses        2563      2075      -488
- Partials      1487      1205      -282

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