feat: client interceptor invocations

[yarolegovich]

Details

The PR implements CallInterceptor invocations and adds tests which demonstrate how interceptors can be used to observe, modify and reject requests and responses.

CallInterceptor can use CallContext to understand which method is being called and be able to extract a typed value from Request.Payload.

CallInterceptor can modify Request.CallMeta to attach information like auth headers to the request context. Transport can access the aggregated CallMeta and add it to the request in a transport-specific way (eg. HTTP headers).

re #47

[gemini-code-assist]

Summary of Changes

Hello @yarolegovich, 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 enhances the A2A client library by introducing a robust client-side interceptor mechanism. This feature allows developers to inject custom logic before and after API calls, enabling advanced functionalities like request/response transformation, authentication, logging, and error handling without modifying the core client logic. The changes also include the addition of standard A2A protocol error types and a comprehensive test suite to ensure the reliability and correctness of the new interceptor system.

Highlights

• Client Interceptor Implementation: Introduced CallInterceptor invocations to allow for observing, modifying, and rejecting client requests and responses.
• Contextual Information: CallInterceptor can now access CallContext to determine the method being called and extract typed payload from Request.Payload.
• Metadata Management: Interceptors can modify Request.CallMeta to attach additional information, such as authentication headers, which can then be used by the transport layer.
• Enhanced Error Handling: Added new A2A protocol-specific error types (ErrParseError, ErrInvalidRequest, ErrMethodNotFound, ErrInvalidParams, ErrInternalError, ErrServerError) to a2a/errors.go.
• Comprehensive Testing: A new test file a2aclient/client_test.go has been added with extensive tests covering various interceptor functionalities, including request/response modification, rejection, and context data sharing.

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[gemini-code-assist]

Code Review

This pull request introduces client-side interceptor invocations, which is a great feature for observing and modifying requests and responses. The implementation is mostly solid, but I've found a critical bug in the error handling logic. Errors returned from the transport layer are currently being swallowed if an interceptor doesn't explicitly return an error. This could lead to silent failures. I've provided detailed comments and suggestions to fix this issue in a2aclient/client.go for each affected method. Additionally, I've suggested adding a new test case to a2aclient/client_test.go to cover this scenario and prevent future regressions. Once these issues are addressed, the implementation will be much more robust.