[FLINK-40120][table] Persist RowData field names in RowDataSerializerSnapshot

[weiqingy]

What is the purpose of the change

This pull request persists the top-level field names of a RowData state serializer in RowDataSerializerSnapshot, so they survive a save/restore round-trip. Today the snapshot stores the field types but not the names.

It is a self-contained, shared building block: the names are needed by schema-aware consumers — the State Catalog (FLIP-599) to expose a readable schema for keyed state, and name-based schema evolution (FLIP-527). They are written unconditionally and decoupled from any evolution logic, so whichever effort lands first establishes a single snapshot format.

Brief change log

• Add a nullable top-level String[] fieldNames to RowDataSerializer, threaded through duplicate() and the snapshot round-trip; equals/hashCode are unchanged (names derive from the types).
• Bump RowDataSerializerSnapshot from V3 to V4, appending the names block after the existing types / nested-serializer section. readSnapshot reads the names only for readVersion >= 4, so V4 readers still restore V3 snapshots (names absent -> null).
• InternalSerializers builds the ROW serializer with its top-level field names (split from the structured-type case, which stays name-less).

Verifying this change

This change added tests and can be verified as follows:

• V4 snapshot round-trip preserves the top-level names, and nested ROW field names remain recoverable from the child logical types.
• Genuine V3 -> V4 backward compatibility: a hand-crafted V3-layout snapshot restores with fieldNames == null and the serializer still round-trips.
• InternalSerializers.create(<a RowType>) yields a serializer whose snapshot carries the names unconditionally.
• duplicate() preserves the names.
• resolveSchemaCompatibility is unchanged: a field-type-array difference still returns incompatible() regardless of names.

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: yes — RowDataSerializer / RowDataSerializerSnapshot, snapshot format V3 -> V4
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: yes — the RowData state snapshot format moves V3 -> V4. Backward compatible: V4 readers restore V3 snapshots (names absent -> null); no runtime behavior change.
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no — it persists metadata not yet consumed by any decision; no user-facing behavior change.
• If yes, how is the feature documented? not applicable

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

• Yes (Claude Code, Claude Opus 4.8)

[flinkbot]

CI report:

• 9d50178 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[spuru9]

LGTM

[raminqaf]

LGTM! Left a nit @weiqingy

[raminqaf]

nit: can be simplified:

Suggested change

	if (readVersion >= 4) {
	if (in.readBoolean()) {
	if (readVersion >= 4 && in.readBoolean()) {

[weiqingy]

Updated, combined into the single short-circuit condition. Thanks!

[gyfora]

LGTM, thank you!