[python] Add create_tag_from_timestamp to FileStoreTable

[JunRuiLee]

Summary

• Add create_tag_from_timestamp(tag_name, timestamp_millis, ignore_if_exists) method to FileStoreTable
• Uses the existing SnapshotManager.earlier_or_equal_time_mills() binary search to find the latest snapshot at or before the given timestamp
• Aligns with Java's CreateTagFromTimestampProcedure behavior

Motivation

PyPaimon currently supports creating tags from a specific snapshot ID or the latest snapshot, but lacks the ability to create a tag from a timestamp — a feature available in the Java API via CreateTagFromTimestampProcedure. This is useful for batch workflows that need to pin a snapshot based on wall-clock time.

Tests

• No new tests added in this PR (existing earlier_or_equal_time_mills is already covered by SnapshotManager tests)

[JingsongLi]

Clean and concise implementation. A couple of minor points:

1. Consistency with Java API: In the Java Paimon TagManager, create_tag_from_timestamp typically uses SnapshotManager.earlierOrEqualTimeMills() which returns a Snapshot or null. Your implementation follows the same pattern. LGTM.

2. Error message: The ValueError message says "earlier than or equal to" but the method name says earlier_or_equal_time_mills. Consider aligning: either say "at or before" or "earlier than or equal to" — just pick one style.

3. Suggest adding a test: Even though this is a thin wrapper, a unit test that creates a couple of snapshots with known timestamps and then calls create_tag_from_timestamp would protect against regressions (e.g., if earlier_or_equal_time_mills semantics change).

Otherwise LGTM — this is a useful addition for time-based tag creation from the Python API.

[JunRuiLee]

@JingsongLi Thanks for the review!

1. Consistency with Java API: Thanks, glad the pattern aligns.

2. Error message: Updated to use "at or before" consistently (matching the docstring).

3. Test: Added test_create_tag_from_timestamp (creates 2 snapshots, tags from a future timestamp, asserts correct snapshot) and test_create_tag_from_timestamp_no_snapshot_raises (timestamp 0, asserts ValueError).