fix: Add support for required params

Merge https://github.com/google/adk-python/pull/2212

This PR closes issue #2202

ADK was not parsing the required attribute when using LiteLLM, letting the LLM decide what is required vs not, not respecting function definitions.

## Test Plan

There's a fork of adk-python that is being running live for over 2 weeks in our production environment with millions of requests per day.

Below you can find a screenshot of the unit tests passing. I've also added one change to the test cases to cover this scenario

<img width="1904" height="483" alt="image" src="https://github.com/user-attachments/assets/5a6eb069-63ae-45a3-baca-6b01543f56fb" />

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2212 from thiagosalvatore:main 7de4037d8016389313f3fb22df40c12bac578523
PiperOrigin-RevId: 797393698

tests/unittests/models/test_litellm.py

@@ -558,8 +558,10 @@ function_declaration_test_cases = [
                             "nested_key1": types.Schema(type=types.Type.STRING),
                             "nested_key2": types.Schema(type=types.Type.STRING),
                         },
+                        required=["nested_key1"],
                     ),
                 },
+                required=["nested_arg"],
             ),
         ),
         {
@@ -581,8 +583,10 @@ function_declaration_test_cases = [
                                 "nested_key2": {"type": "string"},
                             },
                             "type": "object",
+                            "required": ["nested_key1"],
                         },
                     },
+                    "required": ["nested_arg"],
                 },
             },
         },