[FLINK-40208][runtime] Add test coverage and blank-key validation for JobMdcRegistry

[Izeren]

What is the purpose of the change

This is a follow-up to FLINK-40208, which introduced JobMdcRegistry for config-driven MDC enrichment in Flink jobs. This PR adds missing test coverage for three execution paths not exercised by the base PR, and adds a blank-key guard to MdcUtils.

Brief change log

• Added a couple of tests reported on original PR.
• Resolved doc priority conflict.
• Added check for blank keys in configuration with warning.

Verifying this change

• Unit tests

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: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no (test coverage and defensive guard for FLINK-40208)
• If yes, how is the feature documented? not applicable

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

• Yes (please specify the tool below)

Generated-by: Claude Code (claude-sonnet-4-6)

[Izeren]

@rkhachatryan, I have added more tests and minor improvements as a follow-up, PTAL when you have time.
This is based on feedback from @wenshao

[flinkbot]

CI report:

• 70e23e2 Azure: PENDING

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