fix: use official A2A SDK for spec-compliant client implementation

[bokelley]

Fix A2A Adapter by Using Official A2A SDK

Summary

Fixes the A2A protocol adapter implementation by using the official a2a-sdk client instead of a custom HTTP implementation. This resolves response parsing issues and ensures spec compliance.

Changes

1. Refactor to Official SDK (fix: refactor A2A adapter to use official a2a-sdk client)

Adapter Changes:

• Use A2AClient and A2ACardResolver from a2a-sdk package
• Replace manual HTTP requests with official SDK methods
• Use A2A SDK types (Task, Message, TextPart, DataPart, Role, etc.)
• Fix TaskState enum usage (lowercase strings: "completed", "failed")
• Fix Pydantic attribute access (snake_case: task.context_id not task.contextId)
• Handle Part union type wrapper (access via .root)

Test Updates:

• Add helper functions for creating mock A2A responses
• Update all 12 A2A tests to use SDK types
• Mock _get_a2a_client instead of HTTP layer
• All 27 protocol tests passing (12 A2A + 15 MCP)

Benefits:

• ✅ Spec-compliant: Uses canonical A2A client implementation
• ✅ Maintainable: No custom HTTP client code to maintain
• ✅ Type-safe: Leverages A2A SDK Pydantic models
• ✅ Future-proof: Auto-updates with A2A spec changes

2. CI Fixes (fix: resolve ruff and mypy errors in A2A adapter)

Fixed linting and type checking errors:

• Removed unused imports (SendMessageSuccessResponse, TaskState)
• Added missing imports (MessageSendParams, Part, Role)
• Fixed Message construction to use snake_case field names
• Wrapped TextPart in Part union type as required by SDK
• Used Role.user enum instead of string literal
• Fixed line length by extracting variable
• Fixed AgentCard.extensions access with getattr for runtime safety
• Fixed debug_request type annotation

All ruff and mypy checks pass ✓

3. Response Handling Fix (fix: unwrap SendMessageResponse RootModel in A2A adapter)

Critical Bug Fix:

The A2A SDK's SendMessageResponse is a Pydantic RootModel that wraps a union of JSONRPCSuccessResponse | JSONRPCErrorResponse. The adapter was trying to access .error or .result directly on the wrapper, which doesn't have these attributes.

Solution:

sdk_response = await a2a_client.send_message(request)
# Unwrap the RootModel to get the actual response object
response = sdk_response.root if hasattr(sdk_response, "root") else sdk_response

This enables proper response handling for both success and error cases.

Testing

Verified against test agent at https://test-agent.adcontextprotocol.org:

Without Authentication:

• ✅ get_info() - Successfully retrieves agent metadata
• ✅ list_tools() - Successfully lists 16 available tools
• ⚠️ Tool calls return proper authentication errors

With Bearer Token Authentication:

• ✅ get_products() - Successfully retrieved 2 products with full data
• ✅ Authentication header correctly set: Authorization: Bearer <token>
• ✅ Response time: ~247ms

CI Checks:

• ✅ All ruff linting checks pass
• ✅ All mypy type checks pass
• ✅ All protocol tests pass (300/307 total tests pass, 7 CLI test failures are pre-existing)

Release Type

This is a patch release (2.12.1 → 2.12.2) containing bug fixes to make the A2A adapter work correctly with the official SDK.