feat(runtime): add openai-responses provider and Responses API routing

[mxyhi]

Summary

• add a new provider alias: openai-responses
• route OpenAI driver to /v1/responses when model format requires Responses API
• allow explicit force format via openai-responses/<model> prefix
• support OPENAI_REASONING_EFFORT (low|medium|high|xhigh) and model suffix fallback
• update docs for provider/configuration usage

Implementation

• drivers/mod.rs: register openai-responses and include it in known provider hints
• drivers/openai.rs: add Responses request/response structs and routing logic
• agent_loop.rs: prepend openai-responses/ to api_model when provider is openai-responses

Tests

• cargo check -p openfang-runtime
• cargo test -p openfang-runtime drivers::openai::tests:: -- --nocapture
• cargo test -p openfang-runtime test_known_providers_list -- --nocapture