Write Comprehensive Integration Tests

We need to add a new test file to the `tests/` directory that verifies the entire end-to-end function calling workflow is working correctly.

**Acceptance Criteria:**
- A test is created that sends an initial request with a tool definition.
- The test asserts that it receives a `tool_calls` response.
- The test then sends a second request, including the original history and a new `role: "tool"` message with a mocked result.
- The test asserts that the final response is a standard text message that correctly uses the information from the tool result.