feat!: Unify spec domain classes as records

[kabir]

Getter signatures are now changed.

Errors are not converted since they need to extend RuntimeException etc. Similarly the Request/Response objects are not converted either, as we are evaluating moving them out of the spec package. However, first we need to double check if they are still used in the client facing API.

Fixes #507 🦕

Replaces: #509

[gemini-code-assist]

Summary of Changes

Hello @kabir, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly modernizes the codebase by refactoring numerous domain classes into Java records. This change streamlines data access patterns, enhances immutability, and reduces boilerplate code, leading to cleaner and more maintainable code. The update required extensive modifications across client, server, and test modules to adapt to the new record accessor syntax, ensuring consistency and leveraging modern Java features.

Highlights

• Domain Class Unification: Several core domain classes within the spec package, including APIKeySecurityScheme, DataPart, FilePart, HTTPAuthSecurityScheme, Message, MutualTLSSecurityScheme, OAuth2SecurityScheme, OpenIdConnectSecurityScheme, Task, TaskArtifactUpdateEvent, TaskStatusUpdateEvent, and TextPart, have been refactored into Java records. This change simplifies these data-carrying classes by automatically generating boilerplate code like constructors, getters, equals(), hashCode(), and toString() methods.
• Getter Signature Changes: As a direct consequence of converting to records, all getter method signatures have changed from the traditional getFieldName() style to the more concise fieldName() accessor style. This impacts numerous files across the codebase where these domain objects are accessed.
• Immutability Enhancements: The conversion to records inherently promotes immutability for these domain classes, making them safer for concurrent use and easier to reason about. Defensive copying has been applied where necessary for collections within records (e.g., Message parts, Task history and artifacts).
• Selective Conversion: Error classes and Request/Response objects were intentionally not converted to records. Error classes need to extend RuntimeException, which is not compatible with records. Request/Response objects are currently being evaluated for potential relocation outside the spec package, pending a review of their client-facing API usage.
• Serialization Workaround: A temporary workaround has been implemented in JsonUtil.java to explicitly add the kind field during JSON serialization for Part and StreamingEventKind objects. This addresses a current limitation and is tracked for a more permanent fix in issue [Feat]: Remove 'kind' field discriminator #544.

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[gemini-code-assist]

Code Review

This pull request is a great step towards modernizing the codebase by converting many of the spec domain classes to Java records. The changes are extensive and have been applied consistently across the codebase, including updating all call sites to use record accessors instead of getters. The move to immutable records improves the robustness and clarity of the domain model. The review identified a minor issue in a test file where an assertion became redundant after the refactoring. Overall, this is a high-quality refactoring.