fix(table): validate pos delete position chunks

[fallintoplace]

Summary

• replace unchecked positional-delete pos casts with a shared validated collector
• return ErrInvalidSchema when a delete file exposes a non-int64 pos chunk
• add regression coverage for malformed pos chunk types

Why

The Arrow scanner assumed every positional-delete pos chunk was *array.Int64 and cast it directly in both scan paths. If a malformed delete file carried the wrong Arrow type, the scanner panicked instead of returning a schema error.

Testing

• go test ./table -run 'Test(GroupPosDeletesByFilePathSupportsStringLayouts|GroupPosDeletesByFilePathRejectsUnsupportedFilePathLayout|CollectPosDeletePositionsRejectsUnsupportedPosType|ProcessPositionalDeletesAcrossBatches)$' -count=1
• go test ./table -count=1

[tanmayrauth]

LGTM, nice catch turning the blind pos cast into a clean schema error. Just one nit and a question, details inline.

[tanmayrauth]

Is the nil-chunk guard meant purely as defensiveness? groupPosDeletesByFilePath only ever stores retained, non-nil chunks into the map, so this branch shouldn't be reachable from the scan path — just want to make sure I'm not missing a caller that can pass a nil chunk in.

[tanmayrauth]

The inner arr.(*array.Int64) assertion is unreachable here — once the outer chunk.DataType().ID() == arrow.INT64 check at line 401 passes, every chunk of that column is already *array.Int64, so the
!ok branch and its separate "unsupported pos chunk array type" message can't fire. Not a problem to leave as defensive code, but you could drop the , ok check (or fold the two error messages into one) to
keep it tight.