test: Add Some Spark Client Tests and Update Documentation on Generic Tables

[adam-christian-software]

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 security@apache.org)
• [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

[adam-christian-software]

@gh-yzou & @flyrain - Here's the docs update for limitation around credential vending: https://apache-polaris.slack.com/archives/C084QSKD6S2/p1762453063740029?thread_ts=1762203273.837449&cid=C084QSKD6S2

I know that @gh-yzou did a good job at answering Abed's question, so potentially, we can build on this with Yun's answers there.

[gh-yzou]

@adam-christian-software Thanks for help making the doc better! Overall LGTM, got some minor points, it would be great to take care of them before merge! Thanks!

[dimas-b]

As far as I know the Generic Tables API, these changes LGTM (with minor comments). I'll defer to @gh-yzou for approving as my knowledge about the implementation specifics is not complete 😅