Add data length validation for write_section and write_region

[simonrw]

Summary

• write_section used assert! which panics on insufficient data — replaced with Err return
• write_region had no data length validation at all, risking out-of-bounds reads when the data slice was too small for the specified region
• Added proper Err returns with descriptive messages for both

Stacked on #433

Test plan

• All existing tests pass
• Verify write_section returns Err instead of panicking on short data
• Verify write_region returns Err on short data

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 37.50000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.3%. Comparing base (4ba6af5) to head (274c0ba).

Files with missing lines	Patch %	Lines
fitsio/src/images.rs	37.5%	10 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
fitsio/src/images.rs	94.7% <37.5%> (-2.0%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.