feat(spec): Update A2A types from specification 🤖

[a2a-bot]

This PR updates src/a2a/types.py based on the latest specification/json/a2a.json from a2aproject/A2A.

BEGIN_COMMIT_OVERRIDE
fix(spec): Add Transport enum to specification (a2aproject/A2A#909)
Adds Enum of supported Transports to Specification, with extensibility
for proprietary protocols.
Adds URL formatting requirement for AgentInterface.url and
AgentCard.url
END_COMMIT_OVERRIDE

[gemini-code-assist]

Summary of Changes

Hello @a2a-bot, 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 synchronizes the Python A2A data types with the latest external specification. The changes primarily focus on refining the AgentInterface and AgentCard models, enhancing their robustness through explicit field definitions and providing comprehensive documentation to guide implementers on transport protocol handling and URL configurations.

Highlights

• Enhanced Data Model Definitions: Updated AgentInterface and AgentCard models to leverage Pydantic's Field for improved validation, default values, and example data, aligning with the latest A2A specification.
• Standardized Transport Protocols: Introduced a new TransportProtocol Enum to provide explicit, type-safe definitions for supported A2A communication protocols (JSONRPC, GRPC, HTTP+JSON).
• Improved Documentation and Guidance: Expanded docstrings across several fields, including AgentInterface.transport, AgentInterface.url, AgentCard.additional_interfaces, AgentCard.preferred_transport, and AgentCard.url, to offer clearer explanations, best practices, and crucial constraints for A2A agent implementations and client interactions.

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[gemini-code-assist]

Code Review

This pull request updates the A2A Pydantic models in src/a2a/types.py to align with the latest specification. The changes add more detail and examples to the models. The TransportProtocol enum should be used consistently for transport-related fields to improve type safety and maintainability.

[gemini-code-assist]

Consider using the TransportProtocol enum here for the transport field instead of str. This provides better type safety and validation.

Suggested change

	transport: str = Field(..., examples=['JSONRPC', 'GRPC', 'HTTP+JSON'])
	transport: TransportProtocol = Field(..., examples=[TransportProtocol.jsonrpc, TransportProtocol.grpc, TransportProtocol.http_json])

[gemini-code-assist]

The preferred_transport field should use the TransportProtocol enum for type safety and consistency. The default value should also be updated to use the enum member.

Suggested change

	preferred_transport: str | None = Field(
	default='JSONRPC', examples=['JSONRPC', 'GRPC', 'HTTP+JSON']
	)
	preferred_transport: TransportProtocol | None = Field(
	default=TransportProtocol.jsonrpc, examples=[TransportProtocol.jsonrpc, TransportProtocol.grpc, TransportProtocol.http_json]
	)