fix: make LiteLLM streaming truly asynchronous

Merge https://github.com/google/adk-python/pull/1451

## Description
Fixes https://github.com/google/adk-python/issues/1306 by using `async for` with `await self.llm_client.acompletion()` instead of synchronous `for` loop.

## Changes
- Updated test mocks to properly handle async streaming by creating an async generator
- Ensured proper parameter handling to avoid duplicate stream parameter

## Testing Plan
- All unit tests now pass with the async streaming implementation
- Verified with `pytest tests/unittests/models/test_litellm.py` that all streaming tests pass
- Manually tested with a sample agent using LiteLLM to confirm streaming works properly

# Test Evidence:
https://youtu.be/hSp3otI79DM

Let me know if you need anything else from me for this PR

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/1451 from avidelatm:fix/litellm-async-streaming d35b9dc90b2fd6fad44c3869de0fda2514e50055
PiperOrigin-RevId: 774835130

src/google/adk/models/lite_llm.py

@@ -679,7 +679,7 @@ class LiteLlm(BaseLlm):
       aggregated_llm_response_with_tool_call = None
       usage_metadata = None
       fallback_index = 0
-      for part in self.llm_client.completion(**completion_args):
+      async for part in await self.llm_client.acompletion(**completion_args):
         for chunk, finish_reason in _model_response_to_chunk(part):
           if isinstance(chunk, FunctionChunk):
             index = chunk.index or fallback_index