fix table clone visibility for repeated alter

[gouhongshen]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

fixes #24762

What this PR does / why we need it:

Repeated ALTER TABLE statements in one transaction can clone from a table or hidden index table that was produced by an earlier ALTER in the same transaction. The old TableMetaReader only read the committed partition snapshot and constructed the local reader without the current transaction write offset, so the second clone could miss uncommitted object stats from the first clone. The base table data remained visible, but secondary index data could be missing after commit.

This PR keeps the clone path and makes the clone source reader transaction-visible:

• capture the TableMetaReader transaction write offset
• include uncommitted data and tombstone object stats when collecting clone metadata
• use NewLocalDataSource with the same txnOffset for appendable/in-memory clone reads
• include row-level uncommitted tombstones when collecting tombstone rows
• reuse a common helper for collecting uncommitted data/tombstone object stats

It also extends git4data clone BVT coverage with repeated ALTER cases, including a stress case with secondary, unique, fulltext, and ivfflat vector indexes plus multiple ALTER operations in one transaction.

Validation:

• make build
• clone_in_alter_table_2: 81/81 passed
• git4data/clone: 1177/1177 passed
• full git4data: 4403/4403 passed
• git diff --check