Function generate_proto 

pub fn generate_proto(resource: &ResourceDefinition) -> String

Generates a .proto file content from a single ResourceDefinition.

The generated proto includes:

• A message type for the resource with all schema fields
• Create/Update input messages based on endpoint input fields
• Request/Response wrappers for each endpoint
• A gRPC service with RPCs for each declared endpoint
• Server-streaming RPC for list endpoints