Description

FITS binary tables are limited to 999 columns. Writing a larger Table currently reaches HDU construction before that limit is reported, which can emit non-standard TTYPE1000/TFORM1000 warnings before ending with the generic message that TFIELDS is invalid.

This change checks the encoded table's physical column count before constructing the HDU and raises a specific VerifyError. The check happens after mixin encoding so expanded mixin columns are included.

The regression tests cover both sides of the boundary: 999 columns still write successfully, while 1000 columns raise the clear error.

Fixes #19236

Validation

• python -m pytest astropy/io/fits/tests/test_connect.py -q (176 passed, 8 skipped)
• python -m pytest astropy/io/fits -q -n 4 (1661 passed, 39 skipped, 100 xfailed)
• python -m pre_commit run --files astropy/io/fits/connect.py astropy/io/fits/tests/test_connect.py
• Built a Windows CPython 3.13 wheel and installed it into a clean virtual environment
• From that installed wheel, wrote and read back a 999-column FITS file; a 1000-column write raised the new VerifyError and left no output file

AI disclosure

OpenAI Codex assisted with issue triage, implementation, and local validation. The contributor remains responsible for the submitted content and will engage with review feedback.

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