Fix Google Gemini OpenAI compatibility issues (#268)

Gemini is ready to use with this configuration.
```
OPENAI_API_KEY="gemini_key"
OPENAI_BASE_URL="gemini_openai_compatible_url"
CAI_MODEL=openai/gemini-2.5-pro
```

src/cai/sdk/agents/models/openai_chatcompletions.py

@@ -12,6 +12,7 @@ from collections.abc import AsyncIterator, Iterable
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, Literal, cast, overload
 
+import uuid
 import litellm
 import tiktoken
 from openai import NOT_GIVEN, AsyncOpenAI, AsyncStream, NotGiven
@@ -969,6 +970,15 @@ class OpenAIChatCompletionsModel(Model):
                 if not hasattr(self, "_pending_tool_calls"):
                     self._pending_tool_calls = {}
 
+
+                # Fix Google Gemini OpenAI compatibility issues.
+                # When using the OpenAI-compatible API to call tools with Google Gemini
+                # tool_call.id is returned as an empty string.
+                if "openai/gemini" in os.getenv("CAI_MODEL"):
+                    for tool_call in assistant_msg.tool_calls:
+                        if tool_call.id is None or tool_call.id == "":
+                            tool_call.id = uuid.uuid4().hex[:16]
+
                 for tool_call in assistant_msg.tool_calls:
                     # Handle empty arguments before storing
                     tool_args = tool_call.function.arguments