feat: introduce TransportProtocol validation

[martimfasantos]

This pull request improves the way transport protocols are used by some of the classes in A2A agent types by updating relevant fields to use it instead of plain strings.

Transport protocol type safety improvements:

• Updated the transport field in the AgentInterface class to use the new TransportProtocol enum instead of a string, including updating example values.
• Updated the preferred_transport field in the AgentCard class to use the new TransportProtocol enum, set its default to TransportProtocol.jsonrpc, and updated example values.

[gemini-code-assist]

Summary of Changes

Hello @martimfasantos, 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 significantly improves the type safety and consistency of transport protocol definitions within the A2A agent types. By refactoring relevant fields to use a newly introduced TransportProtocol enum instead of plain strings, the changes ensure that all supported protocols are standardized and validated across the codebase, leading to more robust and maintainable agent configurations.

Highlights

• New Enum for Transport Protocols: A new TransportProtocol enum has been introduced to standardize the representation of supported A2A transport protocols, including JSONRPC, GRPC, and HTTP+JSON.
• Type Safety for AgentInterface: The transport field within the AgentInterface class has been updated to use the new TransportProtocol enum, replacing the previous string type, thereby enhancing type safety and validation.
• Type Safety and Default for AgentCard: The preferred_transport field in the AgentCard class now also utilizes the TransportProtocol enum, with its default value explicitly set to TransportProtocol.jsonrpc.

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[gemini-code-assist]

Code Review

This pull request is a great step towards improving type safety by introducing the TransportProtocol enum and using it in AgentInterface and AgentCard instead of raw strings. The refactoring to move the enum definition to a more logical place is also a good cleanup.

I've added a few low-severity suggestions to align the new enum member names with common Python conventions for constants (UPPER_SNAKE_CASE), which should improve readability.

One thing to consider is that the tests in tests/test_types.py don't seem to be updated to reflect these type changes. While the existing tests might still pass due to Pydantic's type coercion, it would be beneficial to add or update tests to explicitly validate the new TransportProtocol enum usage, including default values and handling of invalid inputs.

Overall, this is a solid improvement.