[FLINK-40122][table] Expose a public getter for the persisted RowData field names

[weiqingy]

What is the purpose of the change

This pull request adds a public getter on RowDataSerializerSnapshot that returns the top-level RowData field names persisted by FLINK-40120, so external consumers can read them. It returns null for legacy snapshots (version 3 and earlier) written before field-name persistence.

Note: this PR stacks on #28717 (FLINK-40120), which is under review. Until #28717 merges, the diff here also contains its changes; the net-new change in this PR is the single public getter (see the latest commit). It will be rebased onto a clean master once #28717 merges.

Brief change log

• Add public @Nullable String[] getFieldNames() on RowDataSerializerSnapshot, returning the persisted top-level field names (null for legacy version-3-and-earlier snapshots).

Verifying this change

This change added tests and can be verified as follows:

• snapshotPublicGetterReturnsPersistedNames — a snapshot taken from a named serializer returns the top-level names through the public getter.
• A restored version-3-layout snapshot's getFieldNames() returns null.

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 getter is on RowDataSerializerSnapshot, whose enclosing RowDataSerializer is @Internal
• The serializers: yes — adds a read-only accessor on RowDataSerializerSnapshot; no change to the serialization format or behavior
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager, Checkpointing, Kubernetes/Yarn, ZooKeeper: no — read-only accessor, no format or restore-behavior change
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable (the getter carries JavaDoc)

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

• Yes (Claude Code, Claude Opus 4.8)

Generated-by: Claude Code (Claude Opus 4.8)

[flinkbot]

CI report:

• e5e0a1b Azure: PENDING

Bot commands

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

[weiqingy]

Closing for now — this stacks on #28717 (FLINK-40120), so the diff here includes that PR's changes. I'll open the getter as a clean PR once #28717 merges; it's tracked by sub-task FLINK-40122. Branch and commit are preserved.