Allow a step to be fatally failed via a RetryArtifact in its output data

[akashdw]

Pull Request type

• Bugfix
• [x ] Feature
• Refactoring (no functional changes, no api changes)
• Build related changes (Please run ./gradlew build --write-locks to refresh dependencies)
• Other (please describe):

NOTE: Please remember to run ./gradlew spotlessApply to fix any format violations.

Changes in this PR

• Adds a RetryArtifact (maestro_retry) that a step can include in its output data with retryable: false; when such a step fails with a user or platform error, it is escalated to FATALLY_FAILED instead of
  being retried.
• When the artifact is absent, or present with retryable: true, behavior is unchanged: the failure flows through the existing retry path.
• This keeps failure classification inside Maestro via the existing output-data channel, no new model field or endpoint, and leaves room to extend the artifact with richer retry/classification controls
  later.

[rdeepak2002]

I noticed that other tests in this module, such as WhileArtifactTest.java and TitusArtifactTest.java, include a testRoundTripSerde that verifies the general serde contract.

Would it make sense to add a similar testRoundTripSerde here for consistency and to cover the round-trip behavior as well?

[rdeepak2002]

Thanks for creating this pr! Left a minor comment