bug(tools): tool_choice=required can synthesize schema-invalid empty arguments

[raullenchai]

Summary

With a real Qwen3.5 server, forced tool choice can return a synthetic tool call with {} arguments even when the selected tool's JSON schema requires fields. The response says finish_reason: "tool_calls", so clients may execute a call that Rapid-MLX already knows is invalid.

Environment

• Local wheel built from main at 93981cf4 (package version 0.11.0)
• Qwen3.5-4B
• Tested both streaming and non-streaming chat completions

Reproduction

Provide one tool:

{
  "name": "add",
  "parameters": {
    "type": "object",
    "properties": {
      "a": {"type": "integer"},
      "b": {"type": "integer"}
    },
    "required": ["a", "b"]
  }
}

Prompt: Call add with a=7 and b=8.

Send the request with tool_choice: "required".

Actual behavior

Both streaming and non-streaming responses returned:

{
  "finish_reason": "tool_calls",
  "tool_calls": [{"function": {"name": "add", "arguments": "{}"}}]
}

The server log shows that parsing produced no tool call and the required-tool fallback synthesized one:

SSE-FORCED-SYNTH ... synthesizing terminal call
tool_choice='required' ... produced no tool_calls; synthesising ...

As a control, the same prompt and schema with tool_choice: "auto" produced the correct {"a":7,"b":8} arguments.

Expected behavior

Rapid-MLX should never report a successful forced tool call whose arguments fail the supplied schema. It should either recover/generate valid arguments (ideally through the constrained path) or fail explicitly so the client cannot mistake the result for an executable tool call.