feat: remoteagent a2a request callbacks

[yarolegovich]

Added BeforeA2ARequestCallback, A2AEventConverter and AfterA2ARequestCallback to remoteagent.A2AConfig.

BeforeA2ARequestCallback allows to modify a request or prevent it by returning a non-nil event or error which becomes the result of agent invocation.

A2AEventConverter allows to implement a custom conversion.

AfterA2ARequestCallback allows to enrich the result of the conversion, overwrite it or reject a response.

Additionally, replaced gRPC transport in tests with JSONRPC because of an easier setup.

[gemini-code-assist[bot]]

Summary of Changes

Hello @yarolegovich, I'm Gemini Code Assist[^1]! 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 flexibility of remote agent interactions by introducing pre- and post-request callback mechanisms within the remoteagent.A2AConfig. These new callbacks empower developers to inject custom logic at critical points in the request-response lifecycle, allowing for advanced request manipulation, response enrichment, and conditional processing. Concurrently, the underlying test infrastructure has been streamlined through a migration to JSONRPC, simplifying future development and testing efforts.

Highlights

• New Request Callbacks: Introduced BeforeA2ARequestCallback and AfterA2ARequestCallback types to remoteagent.A2AConfig. These callbacks allow for intercepting and modifying requests before they are sent to a remote agent, and enriching or overwriting responses after they are received.
• Pre-Request Logic: The BeforeA2ARequestCallback enables custom logic to inspect, log, or modify the outgoing request. It can also be used to prevent the actual remote agent call by returning a non-nil event or error, facilitating caching mechanisms or early exit conditions.
• Post-Response Processing: The AfterA2ARequestCallback is invoked after receiving a response (for every event in streaming responses). This callback provides an opportunity to log responses, collect metrics, perform pre-processing of events, or even completely overwrite or reject the original response.
• Test Infrastructure Refactoring: The test suite's transport mechanism has been migrated from gRPC to JSONRPC. This change simplifies the test setup and improves maintainability by removing gRPC-specific dependencies and configurations.

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