feat(http-client): add Android support and implement SPI for A2AHttpClient

[sherryfox]

Summary of Changes

This PR introduces Android support for the A2A HTTP client, decouples the hardcoded JDK client using the Service Provider Interface (SPI) pattern, and aligns with the upcoming Gson migration by rebasing onto the 0.3-workflows branch.

🌟 Key Highlights

• Android Support: Added a dedicated HTTP client for Android using HttpURLConnection.
• SPI Pattern: Decoupled JdkA2AHttpClient and introduced a factory to resolve the best client automatically.
• Gson Alignment: Removed Jackson annotations from spec records to match the Gson fork approach.

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📦 Detailed Changes

1. HTTP Client Factory & SPI Pattern

Introduced a factory and provider interface to allow pluggable HTTP client implementations via ServiceLoader.

• A2AHttpClientFactory.java: Created to load implementations of A2AHttpClientProvider and select the one with the highest priority.
• A2AHttpClientProvider.java: Introduced as the interface for providers.
• Service Configurations: Added files to register the providers in:
  • http-client/src/main/resources/META-INF/services/io.a2a.client.http.A2AHttpClientProvider
  • http-client/src/main/resources_android/META-INF/services/io.a2a.client.http.A2AHttpClientProvider

2. Decoupling JdkA2AHttpClient

Removed hardcoded dependencies on JdkA2AHttpClient across the codebase.

• JdkA2AHttpClient now implements A2AHttpClientProvider with a default priority of 0.
• Updated the following files to use A2AHttpClientFactory.create() instead of direct instantiation:
  • A2A.java (in client/base)
  • A2ACardResolver.java (in http-client)
  • JSONRPCTransportProvider.java (in client/transport/jsonrpc)
  • RestTransportProvider.java (in client/transport/rest)

3. AndroidA2AHttpClient

• Introduced AndroidA2AHttpClient.java using HttpURLConnection (which does not have forbidden dependencies on Android).
• It implements A2AHttpClientProvider with a priority of 100, making it the preferred choice in Android environments.

4. Alignment with Gson Migration

• Rebased onto ehsavoie/a2a-java:0.3-workflows to support the move to Gson and avoid 0.4 leaks.
• Removed Jackson annotations from spec records (like AgentCapabilities, TaskStatus, etc.) as they are not needed in the Gson environment.

5. Documentation & Fixes

• Documentation: Updated README.md to reflect that the default HTTP client is resolved automatically via the factory rather than being hardcoded.
• Kafka Version Update: Kept the Kafka version at 4.2.0 (from the fork) in examples/cloud-deployment/k8s/02-kafka.yaml to ensure compatibility with the CI environment.

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🧪 Verification Results

• Build: Succeeded with mvn clean install -DskipTests=true.
• Tests: All tests in the http-client module passed successfully (mvn test -pl http-client), including a new test verifying the factory behavior.

Description

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Fixes #793 🦕

[gemini-code-assist]

Code Review

This pull request refactors the HTTP client creation by introducing a ServiceLoader-based factory and adds an Android-specific implementation. It also enhances JSON serialization for several record classes in the spec module by adding @JsonProperty annotations. Feedback indicates that the new AndroidA2AHttpClient is missing specific error handling for 401 and 403 status codes in its asynchronous SSE implementation, which should be added to match the synchronous logic.