Replace unimplemented!() with unreachable!()

[simonrw]

Summary

• Three call sites in FitsFile::open, FitsFile::edit, and NewFitsFile::open used unimplemented!() for the case where cfitsio returns success but a null pointer
• This case should be impossible, so unreachable!() better conveys the intent
• unimplemented!() misleadingly suggested a missing feature

Stacked on #433

Test plan

• All existing tests pass

🤖 Generated with Claude Code

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.5%. Comparing base (4ba6af5) to head (b81f944).

Files with missing lines	Patch %	Lines
fitsio/src/fitsfile.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
fitsio/src/fitsfile.rs	94.8% <0.0%> (ø)

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