feat(provider): add OpenAI and Anthropic request/stream codecs

Implements Provider API codecs verified at the codec seam:
- OpenAI Chat Completions body (stream:true, stream_options:{include_usage:true}, messages/tools/images/system/max_tokens/reasoning)
- Anthropic Messages body (stream:true, system top-level, messages/tools/images/max_tokens/reasoning)
- Tool schemas via toJsonSchema, image handling, system flattening, max_tokens capping, reasoning mapping byte-equivalent to shared helpers
- Streaming: incremental text-delta and single finish with usage for both endpoints, message_delta -> finish equivalence, malformed lines ignored, errors redacted
- Adds tests/provider-codecs.test.ts and wires into test:unit