fix(proto): correct gRPC URL example in AgentInterface

[Tehsmash]

Summary

Fixes the incorrect gRPC URL example in the AgentInterface message of a2a.proto.

Problem: The example showed "https://grpc.example.com/a2a" for a gRPC interface, but gRPC URLs do not use an https:// scheme or a path. The correct format is hostname:port.

Fix:

• Updated the example URL from "https://grpc.example.com/a2a" to "grpc.example.com:443"
• Clarified the field comment to note that the HTTPS URL requirement applies to HTTP-based transports only, while gRPC uses hostname:port format

Fixes #1388

[gemini-code-assist]

Code Review

This pull request updates the documentation for the AgentInterface message in specification/a2a.proto to clarify the expected format of the url field for both HTTP-based and gRPC transports. The review feedback suggests refining the terminology from "URL" to "URL or address" when referring to gRPC endpoints to ensure technical accuracy, as gRPC addresses in the format "hostname:port" lack a scheme.

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[gemini-code-assist]

Since hostname:port is an address/endpoint rather than a URL (as it lacks a scheme), referring to it as 'the URL' for gRPC is technically inaccurate. It would be clearer to refer to it as 'the address' or 'the URL or address' to accommodate both HTTP and gRPC transports.

Suggested change

	// The URL where this interface is available. For HTTP-based transports, must be a valid absolute
	// HTTPS URL in production. For gRPC, the URL should be in the format "hostname:port".
	// Example: "https://api.example.com/a2a/v1", "grpc.example.com:443"
	// The URL or address where this interface is available. For HTTP-based transports, must be a valid absolute
	// HTTPS URL in production. For gRPC, the address should be in the format "hostname:port".
	// Example: "https://api.example.com/a2a/v1", "grpc.example.com:443"

[chopmob-cloud]

Verified this fix independently while tracing the same example. The reference SDK confirms the corrected format: a2a-go's gRPC transport passes AgentInterface.url straight into grpc.NewClient(iface.URL, ...) (a2agrpc/v1/client.go), and a gRPC dial target is host:port, so the current https://grpc.example.com/a2a example would fail as written while grpc.example.com:443 works. Scoping the HTTPS wording to HTTP-based transports is the right accompanying clarification. Hope this helps it land.