Remove brittle changelog architecture test

[anth-volk]

Fixes #3789

Summary

• remove the wording-only test that reads a specific Towncrier fragment
• preserve the behavioral architecture guards for service-owned sessions
• add a changelog fragment for this fix

Root cause

The test assumed changelog.d/3788.changed.md would remain in the repository. Towncrier correctly consumes and deletes that fragment in the generated release commit, so the post-release deployment test run failed with FileNotFoundError despite the architecture behavior remaining valid.

Validation

• .venv/bin/pytest tests/unit/services/test_service_owned_session_architecture.py -q (13 passed)
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.19%. Comparing base (3b3daeb) to head (354903f).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3790   +/-   ##
=======================================
  Coverage   84.19%   84.19%           
=======================================
  Files          86       86           
  Lines        4720     4720           
  Branches      926      926           
=======================================
  Hits         3974     3974           
  Misses        510      510           
  Partials      236      236           

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