Rust: preserve structured JSON-RPC error data

Problem

The Rust SDK converts incoming JSON-RPC errors into copilot::Error without exposing the optional error.data payload. Callers can read the numeric code and message but cannot inspect structured error details without parsing human-readable text.

Reproduction

Return a framed JSON-RPC error for a client call, for example:

{
  "jsonrpc": "2.0",
  "id": "1",
  "error": {
    "code": -32600,
    "message": "The request was rejected",
    "data": {
      "code": "request_blocked",
      "setting": "example_setting"
    }
  }
}

The resulting Rust error should make the structured payload available to the caller.

Expected behavior

• Expose retained data through Error::rpc_data() -> Option<&serde_json::Value> for callers using the shared RPC conversion path.
• Preserve non-null JSON objects, arrays, and scalar values; omitted or null data should return None.
• Preserve existing RPC error kind, numeric code, message, display/debug formatting, and source behavior. Reading data should be explicit, not an automatic addition to formatted logs.

Related work

Existing implementation PR: https://github.com/github/copilot-sdk/pull/2664. This issue tracks that work; it is not a request for a duplicate implementation.