feat(proxy): enable stateless VS Code Responses

[romius]

Summary

Migrates CodeMie-managed GPT-5.5 and GPT-5.6 VS Code models to stateless Responses API mode, preventing load-balanced continuation failures caused by previous_response_id state.

Changes

1. Enabled zeroDataRetentionEnabled, Responses reasoning, and model-specific effort capabilities.
2. Extended encrypted reasoning sanitization to VS Code BYOK requests.
3. Added request-normalizer, catalog, sanitizer, and integration coverage.
4. Updated VS Code proxy and user-facing architecture documentation.
5. Preserved streaming behavior and avoided proxy-side conversation storage.

Impact

VS Code now replays conversation and tool history locally with store: false and no previous_response_id. Encrypted hidden reasoning state is removed between deployments while visible history, reasoning effort, and tool calls are preserved.

Checklist

• Self-reviewed
• Manual testing performed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)