Context

As I was investigating how to support Parquet files using Generic Tables, I performed a few miscellaneous clean-ups:

1. I did some minor README cleanup on the Spark Client. This is to streamline the reading and to incorporate the credential vending item as a known limitation.
2. I added a JavaDoc to DeltaHelper. This was to add clarity on what the class was doing.
3. I added some more test coverage of the Spark Client through PolarisRESTCatalogTest, DeltaHelperTest, and PolarisCatalogUtilsTest. This helped increase the code coverage in the plugin.
4. I updated the user-facing docs for generic-table.md and polaris-spark-client.md. This documents the credential vending limitation and streamlines the content.

Checklist

• [X ] 🛡️ Don't disclose security issues! (contact [email protected])
• [X ] 🔗 Clearly explained why the changes are needed, or linked related issues: Fixes #
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• [Not Needed] 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)

Docs Pictures