feat: Add pluggable A2AHttpClient with Vert.x implementation

[ehsavoie]

Introduce ServiceLoader-based A2AHttpClient client architecture with two implementations:

• JDK HttpClient (core, always available)
• Vert.x WebClient (optional, high-performance)

Changes:

• Create http-client-vertx module for Vert.x implementation
• Add ServiceLoader infrastructure (A2AHttpClientFactory, providers)
• Update dependent modules to use Vert.x client by default
• Add comprehensive tests and usage examples

Users can choose implementation via Maven dependencies. Priority-based selection: Vert.x (100) > JDK (0).

Fixes #583 🦕

[gemini-code-assist]

Summary of Changes

Hello @ehsavoie, 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 HTTP client capabilities by introducing a pluggable architecture. It allows the system to dynamically select the most performant HTTP client available, with a new Vert.x-based client offering high-performance asynchronous operations, while retaining the JDK client as a reliable default. This change provides greater flexibility and potential for performance improvements without requiring explicit configuration from the end-user.

Highlights

• Pluggable HTTP Client Architecture: Introduced a new ServiceLoader-based architecture for A2AHttpClient, allowing for flexible and extensible HTTP client implementations.
• Vert.x WebClient Implementation: Added a high-performance Vert.x WebClient-based implementation, VertxA2AHttpClient, which is now the preferred default when Vert.x dependencies are available.
• JDK HttpClient as Fallback: The existing JdkA2AHttpClient has been integrated into the new architecture as a fallback, ensuring a functional client is always available.
• Automatic Client Selection: Implemented a priority-based selection mechanism where VertxA2AHttpClient (priority 100) is chosen over JdkA2AHttpClient (priority 0) by default.
• Refactoring for Factory Usage: Updated various modules (client/base, client/transport/jsonrpc, client/transport/rest, server-common, http-client) to use the new A2AHttpClientFactory for client instantiation, promoting consistency and modularity.
• New Module and Comprehensive Testing: A new http-client-vertx module was created, complete with unit, integration, and SSE-specific tests to ensure the robustness and correctness of the Vert.x implementation.

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[ehsavoie]

@brasseld It has been quite some time but if you want to take a look at this PR :)

[gemini-code-assist]

Code Review

This pull request introduces a pluggable HTTP client architecture using Java's ServiceLoader, with a new Vert.x implementation and a JDK fallback. The changes are well-integrated across the codebase, updating dependencies, imports, and client instantiation to leverage the new A2AHttpClientFactory. The new Vert.x client and its provider are well-documented and include comprehensive tests. The overall design significantly improves flexibility by allowing users to choose or provide their preferred HTTP client implementation.