refactor(server)!: migrate from Application wrappers to Starlette route-based endpoints for rest

[guglielmo-san]

Description

This PR refactors the rest server implementation to expose Starlette Route components directly (via RestRoutes) instead of requiring full FastAPI or Starlette application wrappers.

Ref #797 🦕

[gemini-code-assist]

Summary of Changes

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This pull request significantly refactors the REST server architecture by moving away from encapsulating REST endpoints within a dedicated FastAPI application wrapper. Instead, it introduces a new utility function, create_rest_routes, which directly generates Starlette Route components. This change allows for more flexible integration of REST endpoints into existing Starlette or FastAPI applications, promoting a more modular and less opinionated server setup. The update also includes minor adjustments to task state and part type handling in the Vertex AI task converter and general cleanup across samples and tests to align with the new routing approach.

Highlights

• REST Server Refactoring: The REST server implementation has been refactored to directly expose Starlette Route components via a new create_rest_routes function, eliminating the need for full FastAPI or Starlette application wrappers.
• Removed A2ARESTFastAPIApplication: The A2ARESTFastAPIApplication class and its related files have been removed, simplifying the application structure and reducing dependencies.
• Updated Task Converter Logic: The vertex_task_converter.py file was updated to use vertexai.types.State instead of vertexai_types.A2aTaskState and vertexai.types.Part instead of google.genai.types.Part, streamlining type handling for task states and parts.
• Standardized URL Paths: Trailing slashes were removed from several URL paths in samples/hello_world_agent.py for consistency.
• JSON-RPC Route Enhancements: The create_jsonrpc_routes function now includes Middleware and Router type hints, imports DEFAULT_RPC_URL, and makes the rpc_url parameter optional with a default value.

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[github-actions]

🧪 Code Coverage (vs 1.0-dev)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/request_handlers/rest_handler.py	93.75%	91.67%	🔴 -2.08%
src/a2a/server/routes/rest_routes.py (new)	—	77.11%	—
Total	91.43%	91.37%	🔴 -0.06%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

This pull request refactors the REST application setup by removing the A2ARESTFastAPIApplication class and introducing a new create_rest_routes function. This change streamlines the creation of REST endpoints, including v0.3 compatibility, and integrates them directly with Starlette applications. The vertex_task_converter.py file was updated to align with vertexai.types for task states and parts, removing google.genai dependencies. Additionally, several test files were updated to reflect these changes. Review comments suggest simplifying the _handle_authenticated_agent_card logic in rest_routes.py and removing an unused Middleware import in test_jsonrpc_routes.py.