fix: preserve HMS table properties during commits (#2927)

<!--
Thanks for opening a pull request!
-->

<!-- In the case this PR will resolve an issue, please replace
${GITHUB_ISSUE_ID} below with the actual Github issue id. -->
<!-- Closes #${GITHUB_ISSUE_ID} -->

# Rationale for this change
HMS-specific table properties (e.g., `table_category` from data
contracts) were lost during PyIceberg commits because `commit_table()`
replaced all HMS parameters instead of merging them.

Fixes: #2926

## Are these changes tested?

Yes. Added `test_hive_preserves_hms_specific_properties()` that:
1. Sets HMS properties via Hive client (simulating external systems)
2. Performs PyIceberg commit
3. Verifies HMS properties are preserved

## Are there any user-facing changes?

Bug fix only - HMS properties set outside PyIceberg are now preserved
during commits. No API changes.

---------

Co-authored-by: Fokko Driesprong <fokko@apache.org>