fix: fix tdim issue

[jamesrafe]

Closes #44

Fix one-dimensional FITS TDIM values such as TDIM=(3). These were parsed as an integer, but BinaryField expects dimensions as a tuple or nothing. The new helper normalizes them to (3,) across all binary-table construction paths and adds tests for the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.77%. Comparing base (870e4ee) to head (11c5a1b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   46.35%   46.77%   +0.41%     
==========================================
  Files          17       17              
  Lines        2550     2555       +5     
==========================================
+ Hits         1182     1195      +13     
+ Misses       1368     1360       -8     

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