[core] Skip schema update when no changes detected

[zhongyujiang]

Purpose

This PR fixes an issue where Paimon creates a new schema version even when there are no actual changes to the schema content.

Previously, when executing schema update operations like setOption("foo", "bar") where the option already had the same value, Paimon would still create a new schema version with an incremented ID. This resulted in unnecessary schema versions being created.

Changes

1. Added sameContent() method to TableSchema to compare schema content while ignoring version and timeMillis
2. Modified commitChanges() to check if new schema has same content as old schema before committing
3. Fixed equals() and hashCode() in TableSchema to include highestFieldId for proper comparison

Tests

Added testNoChangeCommitDoesNotCreateNewSchema() in SchemaManagerTest:

• Creates a table with an option foo=bar
• Calls commitChanges(SchemaChange.setOption("foo", "bar")) with the same value
• Verifies that no new schema version is created (ID remains the same)
• Also tests updateComment with unchanged comment value

[zhongyujiang]

@JingsongLi can you please help review this? Thanks!

[JingsongLi]

I think we should modify generateTableSchema remove useless changes and return a Optional<TableSchema>.

[zhongyujiang]

@JingsongLi Thanks for reviewing.
If we try to track every change to remove useless changes , the code gets way too messy. We’d have to also check for updates on table and column comments. Plus, someone might move a column and then move it back (logically possible)—which basically means no change at all.

So, I think just comparing the final result is better. Calculating the new schema is super cheap, and it keeps the logic a lot simpler. What do you think?

[JingsongLi]

@zhongyujiang I think the changes may not be significant. Can you give it a try?