Responses API: Finish wiring up function tool calls

This finishes the plumbing for function tool call and adds a basic
verification test (that passes for me locally against Llama 4 Scout in
vllm).

Signed-off-by: Ben Browning <bbrownin@redhat.com>

tests/verifications/openai_api/fixtures/test_cases/responses.yaml

@@ -31,6 +31,26 @@ test_response_web_search:
         search_context_size: "low"
       output: "128"
 
+test_response_custom_tool:
+  test_name: test_response_custom_tool
+  test_params:
+    case:
+    - case_id: "sf_weather"
+      input: "What's the weather like in San Francisco?"
+      tools:
+      - type: function
+        name: get_weather
+        description: Get current temperature for a given location.
+        parameters:
+          additionalProperties: false
+          properties:
+            location:
+              description: "City and country e.g. Bogot\xE1, Colombia"
+              type: string
+          required:
+          - location
+          type: object
+
 test_response_image:
   test_name: test_response_image
   test_params: