Add Responses to Chat Completions filter

[franciscojavierarceo]

Part of #213.

Summary

• add the openai_responses_to_chat_completions HTTP filter and config
• gate transformation to POST /v1/responses while leaving other Responses endpoints alone
• add docs, example config, registry wiring, and integration coverage

Stack

• depends on Share Responses body size limits #701
• depends on Add Responses request translation core #702
• depends on Add Responses translation semantics #703

Validation

• cargo test -p praxis-proxy-filter --lib --features ai-inference translation
• cargo test -p praxis-proxy-filter --lib --features ai-inference openai::responses
• cargo test -p praxis-tests-integration openai_responses_to_chat_completions
• make lint