Prevent socket write buffer from being cleared when prematurely

[cbbm142]

Adds a check in cheroot.makefile.BufferedWriter._flush_unlocked to prevent clearing of the write buffer when raw.write() returns None due to a blocked stream. Also limits each write call to SOCK_WRITE_BLOCKSIZE bytes, which was defined but not previously used in this method.

❓ What kind of change does this PR introduce?
Prevent socket write buffer from being cleared when it is blocked

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

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

Resolves #821

❓ What is the current behavior? (You can also link to an open issue here)

#821

❓ What is the new behavior (if this is a feature change)?

N/A

📋 Other information:

This bug is hard to reproduce and spot in production since there are no tracebacks or log messages associated with it. It happens because n can be None, and then del self._write_buf[:n] becomes del self._write_buf[:] which will clear everything left in the buffer.

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• 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 #32774939 | 📁 Comparing d3e8258 against latest (2ffb0ba)

  🔍 Preview build  

7 files changed · ± 7 modified

± Modified

• genindex/index.html
• history/index.html
• pkg/cheroot/index.html
• pkg/cheroot.server/index.html
• pkg/cheroot.ssl.pyopenssl/index.html
• pkg/cheroot.test/index.html
• pkg/cheroot.test.test_makefile/index.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.24%. Comparing base (2ffb0ba) to head (d3e8258).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #822      +/-   ##
==========================================
- Coverage   78.32%   78.24%   -0.08%     
==========================================
  Files          41       41              
  Lines        4788     4809      +21     
  Branches      547      549       +2     
==========================================
+ Hits         3750     3763      +13     
- Misses        900      906       +6     
- Partials      138      140       +2