feat: network mock

[chenjiayi3701-lgtm]

Why this change?

1. Structured response data: Wraps body into a response object and adds a contentType: "application/json" field. This allows the backend to know the MIME type when processing the response.
2. Conditional addition: The response field is only added when the user provides the --body parameter, avoiding passing unnecessary null values.
3. Extensibility: The new structure allows adding more properties to the response object in the future (such as statusCode, headers, etc.) without adding more top-level fields.
4. Backend API alignment: This structure better aligns with common API design patterns for network interception/mock responses. The backend likely expects this nested format to properly handle route interception responses.

[vercel]

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