Replace safe Collectors.toList() usages with Stream.toList()

[bowsii]

Fixes #4757

Summary

Replace usages of Collectors.toList() with Stream.toList() where mutability is not required.

Changes

• Replaced safe Collectors.toList() usages across core, runtime, persistence, plugins, and tests.
• Removed unused Collectors imports.
• Preserved mutable-list and nullable-list use cases.
• Reverted two conversions in AbstractLocalIcebergCatalogTest due to generic type inference differences between Collectors.toList() and Stream.toList().

Validation

• ./gradlew format
• ./gradlew :polaris-runtime-service:compileTestJava
• ./gradlew compileAll

All passed successfully.

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 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
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)

[adutra]

@bowsii you have test failures in org.apache.polaris.persistence.nosql.impl.commits.TestCommitLogImpl.

[github-actions]

This PR is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.