[iceberg] Allow publishing VARIANT columns with Iceberg format version 3

[vbabenkoru]

Final PR of the 3-PR stack for #8972 — closes #8972. Stacked on #9244 and #9245: only the top commit belongs to this PR; the 13 commits below it are the two PRs underneath and will disappear from the diff as they merge.

Purpose

Iceberg supports the VARIANT type from format version 3 onward, but v3 also made row lineage mandatory. Paimon therefore rejected VARIANT columns in the Iceberg compatibility layer for all format versions, because the v3 metadata it produced was not lineage-compliant and GA readers would reject the whole table (#8972).

With #9244 and #9245, Paimon's v3 writer emits complete row lineage (table metadata, manifest lists, manifest entries), so the reason to block VARIANT on v3 is gone. This PR lifts the guard for metadata.iceberg.format-version >= 3:

• Tables with VARIANT columns publish Iceberg metadata when the format version is 3, mapping to the Iceberg variant type.
• Format version 2 keeps the existing clear rejection (with a message pointing at the format-version option), since VARIANT does not exist in v2.

The guard lift is intentionally the last commit of the stack: every v3 table that can publish VARIANT is guaranteed to have carried full row lineage from its first commit.

Tests

• IcebergRowLineageCompatibilityTest#testVariantPublishableWithFormatVersion3: a VARIANT column publishes on v3, the schema maps to Iceberg variant, and lineage fields are present.
• IcebergRowLineageCompatibilityTest#testVariantRejectedWithFormatVersion2: v2 commits with VARIANT still fail with the explicit error.

AI notice

The code is generated using Fable 5 (with reviews from Codex) but has been verified to run on a real cluster with Flink, Paimon, Iceberg, StarRocks and Snowflake.