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 📊

✅ 58593 passed | ❌ 1 failed | ⏭️ 2729 skipped | Total: 61323 | Pass Rate: 95.55% | Execution Time: 162m 46s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -75536
Passed Tests	📉 -71097
Failed Tests	📈 +1
Skipped Tests	📉 -4440

➕ New Tests (1)

View new tests

• test_input_attributes_nonstreaming_chat[messages0-expected_system_instructions0-expected_input_messages0]
  • File: tests.integrations.mistral.test_mistral
  • Status: ❌ Failing

❌ Failed Tests

test_input_attributes_nonstreaming_chat[messages0-expected_system_instructions0-expected_input_messages0]

File: tests.integrations.mistral.test_mistral
Suite: py3.10-mistral-v2.6.0
Error: KeyError: 140424429125632

Stack Trace

tests/integrations/mistral/test_mistral.py:389: in test_input_attributes_nonstreaming_chat
    sentry_init(
tests/conftest.py:349: in inner
    client = sentry_sdk.Client(*a, **kw)
sentry_sdk/client.py:277: in __init__
    self._init_impl()
sentry_sdk/client.py:427: in _init_impl
    self.integrations = setup_integrations(
sentry_sdk/integrations/__init__.py:217: in setup_integrations
    for integration_cls in iter_default_integrations(
sentry_sdk/integrations/__init__.py:44: in iter_default_integrations
    yield getattr(import_module(module), cls)
/usr/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1050: in _gcd_import
    ???
<frozen importlib._bootstrap>:1024: in _find_and_load
    ???
<frozen importlib._bootstrap>:171: in __enter__
    ???
<frozen importlib._bootstrap>:123: in acquire
    ???
E   KeyError: 140424429125632

✅ Patch coverage is 100.00%. Project has 2068 uncovered lines.
❌ Project coverage is 90.07%. Comparing base (9d1c31d) to head (28204ee).

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.28%    90.07%    -0.21%
==========================================
  Files          195       179       -16
  Lines        26195     20819     -5376
  Branches      9758      7350     -2408
==========================================
+ Hits         23649     18751     -4898
- Misses        2546      2068      -478
- Partials      1483      1213      -270

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