Improve invalid header encoding errors

[gorewilliams]

Summary

Improve the error raised when a header value cannot be encoded.

Previously, invalid header values could surface a raw UnicodeEncodeError without indicating which header caused the failure. This change raises a clearer ValueError that includes the offending header name while preserving the original encoding error as the cause.

Regression tests cover both mapping-based and iterable header construction.

Closes #864.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing gorewilliams:fix/header-value-encoding-error (3b08e9c) with main (aaef581)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}