Improve ContentLengthError messages to show expected vs received bytes

[bdraco]

What do these changes do?

This PR improves error messages for ContentLengthError by including both the expected and received byte counts. When a response body doesn't match the Content-Length header, users now get a clear message like "Expected 100 bytes, got 60 bytes" instead of just "Not enough data to satisfy content length header."

The changes affect both the Python and Cython HTTP parsers:

• Added tracking of received bytes in the parsers
• Updated error messages to include expected vs received counts
• Added comprehensive tests to verify the new error messages

Are there changes in behavior for the user?

Yes, but only in error messages. The ContentLengthError exception now provides more diagnostic information:

• Before: ContentLengthError: 400, message='Not enough data to satisfy content length header.'
• After: ContentLengthError: 400, message='Not enough data to satisfy content length header. Expected 100 bytes, got 60 bytes.'

This helps users debug issues more easily, such as in the Home Assistant ONVIF camera issue where the exact mismatch between expected and received bytes was unclear.

Is it a substantial burden for the maintainers to support this?

No. The changes are minimal and focused:

• Simple byte counting in the parsers
• No new APIs or breaking changes
• Only affects error message formatting
• Well-tested with comprehensive test coverage

The implementation is straightforward and maintainable - just tracking bytes received and including that information in error messages.

Related issue number

Related to home-assistant/core#148093 (helps diagnose "Not enough data to satisfy content length header" errors)

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
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    number after creating the PR

    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
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    • .deprecation: A declaration of future API removals and breaking
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    • .contrib: Stuff that affects the contributor experience. e.g.
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      environment.
    • .misc: Changes that are hard to assign to any of the above
      categories.

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    for example:

    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

    Use the past tense or the present tense a non-imperative mood,
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.76%. Comparing base (6deecea) to head (82552e7).
⚠️ Report is 41 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11355   +/-   ##
=======================================
  Coverage   98.76%   98.76%           
=======================================
  Files         129      129           
  Lines       43416    43461   +45     
  Branches     2324     2324           
=======================================
+ Hits        42879    42924   +45     
  Misses        383      383           
  Partials      154      154           

Flag	Coverage Δ
CI-GHA	98.64% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.38% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.81% <100.00%> (-0.01%)	⬇️
OS-macOS	97.69% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.33% <100.00%> (+<0.01%)	⬆️
Py-3.10.18	97.73% <100.00%> (+<0.01%)	⬆️
Py-3.11.13	97.91% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.52% <100.00%> (-0.01%)	⬇️
Py-3.12.10	97.63% <100.00%> (-0.01%)	⬇️
Py-3.12.11	98.03% <100.00%> (+<0.01%)	⬆️
Py-3.13.5	98.27% <100.00%> (-0.01%)	⬇️
Py-3.9.13	97.22% <100.00%> (+<0.01%)	⬆️
Py-3.9.23	97.61% <100.00%> (-0.01%)	⬇️
Py-pypy7.3.16	86.00% <100.00%> (-8.90%)	⬇️
VM-macos	97.69% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	98.38% <100.00%> (+<0.01%)	⬆️
VM-windows	96.81% <100.00%> (-0.01%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #11355 will not alter performance

_{Comparing log_missing_bytes (82552e7) with master (6deecea)}

Summary

✅ 59 untouched benchmarks

[bdraco]

I was able to find the underlying issue which turned out to be broken firmware on a camera without this change. I'll leave it here in case something things its still worth merging

[Dreamsorcerer]

Is there any way we can get the else condition here? My understanding is that it shouldn't be possible. If we receive the number of bytes expected, then we consider that message complete and read any further bytes as the start of the next message. Therefore, we should never be able to have more than the expected number of bytes (I also don't see any equivalent logic in the Cython changes).