Two fixes for OpenAI-compatible provider compatibility:
1. Sanitize JSON Schema `const` → `enum` in tool parameters.
Many OpenAI-compatible endpoints (Ollama, DeepSeek, vLLM, etc.)
do not support the `const` keyword in JSON Schema. Recursively
convert `const: value` to `enum: [value]` which is semantically
equivalent.
2. Force stop_reason to `tool_use` when tool_calls are present.
Some backends incorrectly return finish_reason "stop" even when
the response contains tool_calls. Without this fix, the query
loop treats the response as a normal end_turn and never executes
the requested tools.
