Add OpenAI-compatible backend for local LLM support (GPT-OSS)

Implement OpenAIClient backend that speaks the OpenAI Chat Completions
API (/v1/chat/completions), enabling llm9p to work with any local model
server: Ollama, vLLM, llama-server, LocalAI, or LM Studio.

Primary target is GPT-OSS (OpenAI's open-weight MoE models), but any
model served via these platforms works. The backend supports:
- Blocking and streaming chat completions
- Tool/function calling with STOP:/TOOL: formatting
- Token counting from API usage (with estimation fallback)
- Conversation history, system prompts, temperature control
- Stateless AskWithRequest for session isolation

Usage:
  ./llm9p -backend openai -openai-url http://localhost:11434/v1 -model gpt-oss:20b

Also fixes pre-existing stale mock backends in test files (AskWithRequest
signature was out of date with the Backend interface).

https://claude.ai/code/session_017qVVZUUhfCCvNkMYmXDZAa