[FLINK-39125][table] Support injective casts from BINARY/VARBINARY to CHAR/VARCHAR for upsert keys

[gustavodemorais]

What is the purpose of the change

Recognize CAST from BINARY/VARBINARY to CHAR/VARCHAR as injective so that upsert keys are preserved through such projections. This unblocks upsert key tracking for byte-keyed columns cast to string, now that strict UTF-8 decoding is the default.

Brief change log

• Add WHEN_CHAR_LENGTH_FITS_UTF8 injective rule for BINARY/VARBINARY to CHAR/VARCHAR in LogicalTypeCasts
• Gate the injectivity in FlinkRelMdUniqueKeys on the legacy-bytes-to-string-cast config, since legacy U+FFFD substitution breaks uniqueness
• Thread the input RelNode through getProjectUniqueKeys so the config is reachable

Verifying this change

• LogicalTypeCastsTest
• FlinkRelMdUpsertKeysTest
• FlinkRelMdUniqueKeysTest

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? (no)
• If yes, how is the feature documented? (not applicable)

---

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

• Yes (please specify the tool below)

2.1.181 (Claude Code) with Opus 4.8

[flinkbot]

CI report:

• 851d855 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[twalthr]

Thank you @gustavodemorais for completing the injective cast work with this PR. LGTM