Support validation for method fields in *Request

[zhongyi51]

Closes: #75

The Problem

As identified in issue #75, the individual *Request structs (e.g., PingRequest, ListResourcesRequest) do not currently validate that their method field matches the constant value defined in the JSON schema. This creates ambiguity. For example, a JSON payload like:

{
  "method": "wrong/method",
  "params": { /* ... valid params for ListResourcesRequest ... */ }
}

could be incorrectly deserialized as a ListResourcesRequest. The deserialization process should fail fast if the method name is incorrect for the target struct.

The Solution

This pull request proposes a robust pattern to validate the method field by leveraging Serde's #[serde(deserialize_with = "...")]:

1. Deserializes the incoming value as a string.
2. Compares it against the expected constant value for that specific request type.

Note for Maintainers

⚠️ This pull request should NOT be merged directly. It serves as a working example for the code generator.