Fix observability middleware test isolation

[anth-volk]

Fixes #3856

Summary

This PR removes unrelated deployment configuration assumptions from two Flask observability middleware tests. The assertions continue to verify request metadata and observability_id propagation without requiring a specific simulation entrypoint.

The previous assertions caused deployment run 36602925386 to fail when the staging environment supplied SIM_ENTRYPOINT=cloud_run_simulation_entrypoint.

Changes

• Assert only the request context fields exercised by each middleware test.
• Remove hard-coded simulation entrypoint and database routing values from those assertions.
• Add a changelog fragment for Isolate observability middleware tests from simulation entrypoint configuration #3856.
• Apply Ruff's import ordering to the modified test module.

Testing

• env SIM_ENTRYPOINT=cloud_run_simulation_entrypoint uv run --frozen pytest tests/unit/routes/test_migration_context_logging.py -q — 26 passed
• uv run --frozen ruff format --check tests/unit/routes/test_migration_context_logging.py
• uv run --frozen ruff check tests/unit/routes/test_migration_context_logging.py
• make format

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.65%. Comparing base (ca912a3) to head (35e56d0).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3857       +/-   ##
===========================================
+ Coverage   45.76%   87.65%   +41.89%     
===========================================
  Files         182      197       +15     
  Lines       10812    11952     +1140     
  Branches     1837     2092      +255     
===========================================
+ Hits         4948    10477     +5529     
+ Misses       5456      901     -4555     
- Partials      408      574      +166     

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