Deprecate isUpdateableGeoJSON in favor of cleaner input validation

[wayofthefuture]

This PR removes the isUpdateableGeoJSON() function and consolidates validation and normalization logic into toUpdateable(). Previously, correctness depended on keeping two separate functions in sync, which risked logic drift and duplicated checks. By handling both validation and conversion in a single place, the update pipeline becomes simpler, more predictable, and easier to maintain. The caller now has a single, clear entry point: attempt to build an updateable map, and rely on undefined to signal invalid input.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.41%. Comparing base (a1d1972) to head (d92a72c).

Files with missing lines	Patch %	Lines
src/source/geojson_source_diff.ts	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6819      +/-   ##
==========================================
- Coverage   92.42%   92.41%   -0.01%     
==========================================
  Files         288      288              
  Lines       23814    23807       -7     
  Branches     5058     5055       -3     
==========================================
- Hits        22009    22002       -7     
  Misses       1805     1805              

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[HarelM]

Yeah, I can see how this is cleaner...