feat: Add custom User-Agent header to RestApiTool requests

Merge https://github.com/google/adk-python/pull/2641

This PR adds a custom `User-Agent` header to all requests made via `RestApiTool`. This allows backend services to identify traffic originating from the ADK. The header format is `google-adk/<version> (tool: <tool_name>)`, where `<version>` is the current version of the `google-adk` package, fetched dynamically from `google.adk.version`, and `<tool_name>` is the name of the specific `RestApiTool` instance.

**Associated Issue**

Fixes #2676

**Testing Plan**

**Unit Tests**

I ran the full suite of unit tests locally to ensure the changes did not introduce any regressions. All tests passed successfully.
```bash
$ pytest ./tests/unittests
================================= 4202 passed, 2459 warnings in 44.68s ====================
```
The warnings are related to existing experimental features and are not affected by this change.

**Manual End-to-End (E2E) Test**

I performed a manual test to ensure the integrated flow works as expected.

*   **Setup:** Created a clean virtual environment (`~/venvs/adk-e2e-test`) and installed the locally built `google-adk` package using `uv venv --seed` and `pip install dist/google_adk-*.whl`. Ran a custom `mock_server.py` script (using Flask) in one terminal to listen for requests and print headers. Ran a custom `run_test_tool.py` script in a second terminal to send a request using the modified `RestApiTool`.

*   **Execution:** The `run_test_tool.py` script successfully sent a POST request to the mock server's `/test` endpoint. The mock server received the request and printed the headers.

    **`run_test_tool.py` Output:**
    ```json
    {
      "message": "Request received successfully!"
    }
    ```

    **`mock_server.py` Output (Headers):**
    ```text
    --- Request Received ---
    Headers:
      Host: 127.0.0.1:8000
      User-Agent: google-adk/1.12.0 (tool: TestTool)
      Accept-Encoding: gzip, deflate
      Accept: */*
      Connection: keep-alive
      Content-Type: application/json
      Content-Length: 2
    ------------------------
    ```
    The `User-Agent: google-adk/1.12.0 (tool: TestTool)` header confirms the change is working as intended, dynamically fetching the package version and including the tool name.

**Checklist**

*   [x] Associated issue linked
*   [x] Unit tests passed
*   [x] End-to-end test performed and results documented
*   [x] Code formatted with `./autoformat.sh`

---

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2641 from rcsantana777:feat/add-user-agent a9a9375306c18bb7ba501276cbf76693e70a87ad
PiperOrigin-RevId: 798232385

src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py

@@ -245,6 +245,7 @@ class RestApiTool(BaseTool):
     Example:
         self._prepare_request_params({"input_id": "test-id"})
     """
+
     method = self.endpoint.method.lower()
     if not method:
       raise ValueError("Operation method not found.")
@@ -254,6 +255,12 @@ class RestApiTool(BaseTool):
     header_params: Dict[str, Any] = {}
     cookie_params: Dict[str, Any] = {}
 
+    from ....version import __version__ as adk_version
+
+    # Set the custom User-Agent header
+    user_agent = f"google-adk/{adk_version} (tool: {self.name})"
+    header_params["User-Agent"] = user_agent
+
     params_map: Dict[str, ApiParameter] = {p.py_name: p for p in parameters}
 
     # Fill in path, query, header and cookie parameters to the request