[core] Prevent branch commits from overwriting Iceberg metadata

[ArnavBalyan]

Purpose

• Paimon runs Iceberg callbacks for commits on every branch.
• Non main branch operations use the main table’s Iceberg metadata location, causing main metadata and the external catalog pointer to be overwritten.
• Run Iceberg validation, commit and tag callbacks only for the main branch.

Tests

• UT

[ArnavBalyan]

cc @JingsongLi thanks!

[JingsongLi]

[P1] Refresh Iceberg metadata when a branch is fast-forwarded to main

Disabling these callbacks on non-main branches prevents the premature overwrite, but it also leaves the normal promotion path without any Iceberg refresh. FileSystemBranchManager.fastForward only copies the branch snapshot/schema/tag files into the main directories and invalidates the Paimon snapshot cache; it never runs a main-branch IcebergCommitCallback. Therefore, after table.fastForward("b1"), Paimon main points at the promoted snapshot while the external Iceberg catalog and version-hint.text still point at the old main snapshot indefinitely. The new test stops before promotion, so it does not expose this stale-reader state.

Please make the fast-forward path rebuild/commit Iceberg metadata for the newly promoted main snapshot (while keeping branch commits callback-free), and add a regression that verifies the external pointer advances only after fast-forward.