[FLINK-40225][table] Fixed the Leading token bug

[VasShabu]

What is the purpose of the change

Fixed the bug which causes some invalid JSON elements being parsed correctly without issue.

Brief change log

(for example:)

• Ensured that failing on leading token got enabled
• Corrected the invalid JSON that was present in the JSON_UNQUOTE function

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change added tests and can be verified as follows:
Some tests were added as this PR
./mvnw -o -pl flink-table/flink-table-planner -Dtest='JsonFunctionsITCase' -Dsurefire.failIfNoSpecifiedTests=false -Dcheckstyle.skip=true -Dspotless.check.skip=true -e -Drat.skip=true -Denforcer.skip=true test

• SELECT 'true, false' IS JSON; // False
• SELECT 'true sdfoishjdoifhfs' IS JSON; // False

Previously these functions returned true

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): (yes)
• 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)

[flinkbot]

CI report:

• b6bbd66 Azure: FAILURE

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