Fix unservicable connection error logging

[andrewkernel]

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

Resolves #797

❓ What is the current behavior?

HTTPServer._serve_unservicable() calls self.server.error_log() when HTTPRequest.simple_response() raises an unexpected exception, but HTTPServer does not have a server attribute. That means the 503 handler's error logging path raises another exception instead of logging the original failure.

❓ What is the new behavior?

The 503 handler now calls self.error_log(), matching other HTTPServer logging paths. A focused regression test covers the unexpected-error branch and verifies that the connection is still marked for linger/close.

📋 Other information:

Verification:

• python -m py_compile cheroot/server.py cheroot/test/test_server.py
• .\.venv\Scripts\python.exe -m pytest cheroot/test/test_server.py -k unservicable -q --no-cov
• .\.venv\Scripts\python.exe -m pytest cheroot/test/test_server.py::test_stop_interrupts_serve cheroot/test/test_server.py::test_unservicable_conn_logs_unexpected_response_errors -q --no-cov

The targeted pytest selection also passes under the repository config, but the repo-wide coverage threshold fails when only this single test is selected; the --no-cov runs above verify the regression locally.

📋 Contribution checklist:

• I wrote descriptive pull request text above
• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

[read-the-docs-community]

Documentation build overview

📚 cheroot | 🛠️ Build #33300654 | 📁 Comparing b0177b8 against latest (3937fe1)

  🔍 Preview build  

6 files changed · ± 6 modified

± Modified

• genindex/index.html
• history/index.html
• pkg/cheroot/index.html
• pkg/cheroot.server/index.html
• pkg/cheroot.test/index.html
• pkg/cheroot.test.test_server/index.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.33%. Comparing base (3937fe1) to head (b0177b8).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #830      +/-   ##
==========================================
+ Coverage   78.09%   78.33%   +0.24%     
==========================================
  Files          41       41              
  Lines        4788     4810      +22     
  Branches      547      547              
==========================================
+ Hits         3739     3768      +29     
+ Misses        910      903       -7     
  Partials      139      139