Is your feature request related to a problem? Please describe.

The domain objects in spec are immutable but use either class or record for their definitions.
That makes the api quite inconsistent (eg task.getTaskId() vs artifact.artifactId()).

Use records for the domain object would be more consistent and helps with #506.

This would be a breaking change in the API though...

Describe the solution you'd like

Use records for the domain API so that the API naming conventions and usage would be more consistent.
It should at least be done for the "payloads" (Task, Message, update events, etc.) and the agent card (and its attributes).

Types that would be affected are:

• APIKeySecurityScheme
• DataPart - done in refactor!: Convert domain classes to Java records #509
• FilePart - done in refactor!: Convert domain classes to Java records #509
• HTTPAuthSecurityScheme
• Message
• MutualTLSSecurityScheme
• OAuth2SecurityScheme
• OpenIdConnectSecurityScheme
• Task - done in refactor!: Convert domain classes to Java records #509
• TaskArtifactUpdateEvent
• TaskStatusUpdateEvent
• TextPart - done in refactor!: Convert domain classes to Java records #509

Describe alternatives you've considered

No response

Additional context

No response

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