Use CStr::from_bytes_until_nul for safer string conversion

[simonrw]

Summary

• buf_to_string used CStr::from_ptr which scans for a null terminator without any bounds check — if the buffer is not null-terminated it reads past the end
• Replaced with CStr::from_bytes_until_nul which is bounded by the buffer length and returns an error if no null terminator is found

Test plan

• All existing tests pass

🤖 Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/buf-to-string-null-handling

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.5%. Comparing base (83b76a5) to head (a36aa2a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
fitsio/src/stringutils.rs	95.5% <100.0%> (+0.2%)	⬆️

... and 2 files with indirect coverage changes

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