fix: honor log fallback after correlation ID is cleared

[Gaurang-goyal]

When the correlation ID is explicitly cleared with correlation_id.set(None), CorrelationIdFilter(default_value="-") currently attaches None to the log record. ContextVar.get(default) only uses its fallback when no value has been set, so the cleanup patterns documented for SAQ and Celery bypass the configured fallback. A formatter using {correlation_id:s} then raises TypeError.

Use the configured fallback when the retrieved ID is None. Existing IDs, empty strings, truncation, and the default None behavior are preserved, and the context itself is not changed.

Adds regression coverage for cleared IDs, fallback truncation, empty-string preservation, and formatting a cleared ID with a string format specifier.

Validation (Python 3.12/macOS):

• Three regression cases fail before the fix; all 41 tests pass afterward.
• 95% overall coverage; 100% for the logging filter.
• Ruff 0.15.15 lint/format checks, ty type checking, and all applicable pre-commit hooks pass.

Test dependencies were installed in an isolated environment using current versions. Two existing/dependency warnings concern the Starlette httpx test client and an asyncio marker on a synchronous test. Upstream multi-version CI has not run yet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.91%. Comparing base (76b61b6) to head (624436d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   94.44%   94.91%   +0.47%     
==========================================
  Files           9        9              
  Lines         270      295      +25     
==========================================
+ Hits          255      280      +25     
  Misses         15       15              

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