Fix TransportProtocol comment: JSON-RPC is optional, not mandatory

[Copilot]

The comment in the TransportProtocol enum incorrectly stated that JSON-RPC was "(mandatory)" while gRPC and HTTP+JSON were "(optional)". This contradicted the A2A specification which clearly states that all three transport protocols are equally optional.

According to the specification Section 3.2:

• Section 3.2.1: "Agents MAY support JSON-RPC 2.0 transport"
• Section 3.2.2: "Agents MAY support gRPC transport"
• Section 3.2.3: "Agents MAY support REST-style HTTP+JSON transport"

The specification explicitly states: "All three protocols are considered equal in status, and agents may choose to implement any combination of them based on their requirements."

Before:

export enum TransportProtocol {
  JSONRPC = "JSONRPC", // JSON-RPC 2.0 over HTTP (mandatory)
  GRPC = "GRPC", // gRPC over HTTP/2 (optional)
  HTTP_JSON = "HTTP+JSON", // REST-style HTTP with JSON (optional)
}

After:

export enum TransportProtocol {
  JSONRPC = "JSONRPC", // JSON-RPC 2.0 over HTTP (optional)
  GRPC = "GRPC", // gRPC over HTTP/2 (optional)
  HTTP_JSON = "HTTP+JSON", // REST-style HTTP with JSON (optional)
}

This change ensures the code documentation accurately reflects the specification requirements and prevents confusion for developers implementing A2A-compliant agents.

Fixes #1098.

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