fix: Handle non-indexed function call chunks with incremental fallback index

This is in response to the litellm v1.71.2 + ollama v0.9.0 sending function call chunks with 0 indices across multiple calls and lacking call ids.

Solutions introduced:
1. increment fallback index when accumulated arg becomes json parsable.
2. tolerate finish reason == stop when tool calls are present
3. fallback to index when tool call id is None

Fixes https://github.com/google/adk-python/issues/294

PiperOrigin-RevId: 766258344

src/google/adk/models/lite_llm.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
 
 import base64
 import json
@@ -669,11 +670,11 @@ class LiteLlm(BaseLlm):
       aggregated_llm_response = None
       aggregated_llm_response_with_tool_call = None
       usage_metadata = None
-
+      fallback_index = 0
       for part in self.llm_client.completion(**completion_args):
         for chunk, finish_reason in _model_response_to_chunk(part):
           if isinstance(chunk, FunctionChunk):
-            index = chunk.index or 0
+            index = chunk.index or fallback_index
             if index not in function_calls:
               function_calls[index] = {"name": "", "args": "", "id": None}
 
@@ -681,8 +682,17 @@ class LiteLlm(BaseLlm):
               function_calls[index]["name"] += chunk.name
             if chunk.args:
               function_calls[index]["args"] += chunk.args
+
+              # check if args is completed (workaround for improper chunk
+              # indexing)
+              try:
+                json.loads(function_calls[index]["args"])
+                fallback_index += 1
+              except json.JSONDecodeError:
+                pass
+
             function_calls[index]["id"] = (
-                chunk.id or function_calls[index]["id"]
+                chunk.id or function_calls[index]["id"] or str(index)
             )
           elif isinstance(chunk, TextChunk):
             text += chunk.text
@@ -700,7 +710,9 @@ class LiteLlm(BaseLlm):
                 total_token_count=chunk.total_tokens,
             )
 
-          if finish_reason == "tool_calls" and function_calls:
+          if (
+              finish_reason == "tool_calls" or finish_reason == "stop"
+          ) and function_calls:
             tool_calls = []
             for index, func_data in function_calls.items():
               if func_data["id"]: