refactor(server)!: remove Starlette and FastAPI utilities and rely only on routes

[guglielmo-san]

Description

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[gemini-code-assist]

Summary of Changes

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This pull request streamlines the server's architecture by decoupling it from specific web frameworks like Starlette and FastAPI. It introduces a new, modular routing system that centralizes the definition and handling of A2A protocol endpoints. This change enhances maintainability, improves flexibility for integrating with various ASGI frameworks, and simplifies the overall server implementation by consolidating request dispatching logic.

Highlights

• Architectural Refactoring: The server's HTTP handling has been significantly refactored to remove direct dependencies on Starlette and FastAPI application classes, moving towards a more modular, framework-agnostic approach.
• New Routing Module: A new routes module has been introduced, containing dedicated classes (AgentCardRoutes, JsonRpcRoutes, RestRoutes) that encapsulate the logic for handling specific A2A protocol endpoints.
• Centralized Request Dispatching: The JSONRPCApplication has been renamed to JsonRpcDispatcher and its responsibilities shifted to directly handle request processing, eliminating the need for separate JSONRPCHandler and RESTHandler classes.
• Simplified Application Integration: The new route classes can be easily integrated into any ASGI framework (like Starlette or FastAPI) by providing a list of routes, promoting greater flexibility and reducing framework coupling.
• Removal of Old Application Structures: All files related to the previous a2a.server.apps module, including jsonrpc_app.py, fastapi_app.py, starlette_app.py, and rest_adapter.py, have been removed.

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[gemini-code-assist]

Code Review

The pull request successfully refactors the server's HTTP application components, moving from dedicated FastAPI/Starlette application classes to a more modular route-based structure. This change streamlines the architecture by centralizing route definitions and dispatching logic. Several application and handler files were removed, and new route-specific modules were introduced, along with corresponding updates in imports and test fixtures across the codebase. Overall, the refactor improves modularity and simplifies the integration of HTTP frameworks by relying solely on route definitions.

However, I've identified a few minor issues related to consistency in __all__ exports, error messages, and a potential oversight in passing modifiers to a compatibility adapter. There are also a couple of bugs in the new test files that need to be addressed.