Support simple JSON-based tests for requests

[zhiguang-chen]

Add support for lightweight, declarative tests for each request. Each request can include a tests field with simple JSON assertions (status code, headers, body content, JSON path, etc.).

Expected Behavior:

• Each request config can have a tests array.
• Each test includes a name, type, operator (like equals, includes), and value.
• The system executes these tests automatically after sending the request and reports pass/fail.

Example:

{
  "tests": [
    { "name": "Status is 200", "type": "status", "operator": "equals", "value": 200 },
    { "name": "Body contains username", "type": "bodyContains", "value": "username" }
  ]
}