[FLINK-40493][table] Support casting from VARIANT to UUID

[manner]

What is the purpose of the change

This pull request adds CAST and TRY_CAST from a VARIANT holding a UUID (variant primitive code 20) to the native UUID type, completing UUID coverage for VARIANT under FLIP-604.

A VARIANT whose stored kind is UUID can now be cast to the UUID type, and it renders to a character string in the canonical lower-case 8-4-4-4-12 form like any other scalar kind.

Consistent with the existing VARIANT-cast semantics, a cast never reinterprets the stored kind, so the UUID variant casts only to UUID (and to a string).

Brief change log

• VariantCastUtils.toUuid decodes a UUID-kind variant into the 16-byte big-endian encoding used internally for the UUID type, renderScalar gains a UUID case producing the canonical string.
• VariantToPrimitiveCastRule accepts UUID as a target and generates the toUuid call.
• LogicalTypeCasts declares VARIANT as an explicit source for castTo(UUID).
• Documentation (data-types.md, both content and content.zh) lists UUID in the "stored kind → succeeds for" table for VARIANT casts.

Verifying this change

This change added tests and can be verified as follows:

• CastRulesTest: added a VARIANT → UUID case (success and a TableRuntimeException for a non-UUID kind) and a VARIANT → STRING case asserting the canonical rendering.
• LogicalTypeCastsTest: VARIANT → UUID is explicit-only castable; UUID → VARIANT stays unsupported.
• CastRuleProviderTest: the VARIANT → UUID rule resolves.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude Code (Opus 4.8)

[flinkbot]

CI report:

• f7798dc Azure: SUCCESS
• 9fbe39d Azure: PENDING

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[davidradl]

I assume casting to its existing type is safe - does it warrant a test?

[manner]

There's an identity cast test in line 389: Arguments.of(new UuidType(), new UuidType(), true, true). Did you mean that one?

[raminqaf]

For now this test is fine. For the future we will add support of CAST primitive dataTypes to Variant.

[davidradl]

Approve pending the CI completing and the small comments being addressed.

[raminqaf]

Thanks @manner! I left some nits. Should be good in the next round!

[raminqaf]

I think this piece of code exists in many places for UUID now. Could you double check if we can re-use this method more and replace the duplicates?

[manner]

In the current location only RexLiteralUtil in the planner can reuse it, to deduplicate it also in UuidUuidConverter it would have to move to flink-table-common.

[raminqaf]

Nit: extract UUID.fromString as static field and re-use