fix: convert tool_calls to string when no available_functions provided

[devin-ai-integration]

Summary

Fixes #4036 - Models with native function calling (like Ollama) fail with 'list' object has no attribute 'rstrip' error.

When a model returns tool_calls without text content and no available_functions are provided, the code previously returned the raw tool_calls list. This list would then cause an error when passed to format_message_for_llm() which expects a string.

This PR converts tool_calls to a human-readable string representation containing the tool name and arguments, allowing the agent to see what tool the model wanted to call.

Changes:

• Modified _handle_non_streaming_response to convert tool_calls to string
• Modified _ahandle_non_streaming_response with the same fix
• Added warning log when this conversion occurs
• Added 4 tests covering sync/async paths and edge cases

Review & Testing Checklist for Human

• Test with actual Ollama model: The fix was only tested with mocked responses. Please verify with a real Ollama model that has native function calling enabled (e.g., ollama_chat/gpt-oss:20b as mentioned in the issue)
• Verify string format is useful: The format Tool: {name}\nArguments: {args} is somewhat arbitrary. Confirm this format works well with the agent's text-based tool parsing, or if a different format (e.g., JSON) would be more appropriate
• Check for other code paths: Verify there are no other places in the codebase that might return raw tool_calls lists that could cause similar issues

Recommended test plan:

1. Set up an Ollama model with native function calling support
2. Create a simple agent with tools
3. Run a task that triggers the model to use a tool
4. Verify no crash occurs and the agent can see the tool call information

Notes

• This fix makes the failure mode nicer but does NOT enable native function calling for the standard agent tool loop - it converts tool calls to text so the agent can handle them through text-based parsing
• The broad except Exception catch in the conversion is intentional as a fallback to str(tool_calls) if the tool_call structure is unexpected

Link to Devin run: https://app.devin.ai/sessions/196112f708ab4589be19e3987b8c584c
Requested by: João ([email protected])

[devin-ai-integration]

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