fix(proxy): normalize Responses user identifiers

[romius]

Summary

Fixes Azure Responses API failures caused by project profiles producing user identifiers longer than Azure’s 64-character limit. The fix applies consistently to Codex, OpenCode, CodeMie Code, and VS Code BYOK traffic.

Changes

• Generalized the VS Code-specific normalizer for all supported Responses API clients.
• Adds a short client fallback when user is missing or invalid.
• Preserves short identifiers and replaces long ones with stable 32-character SHA-256 hashes.
• Retains the existing VS Code plugin export for backward compatibility.
• Recalculates Content-Length without logging identity values.

Impact

Before: Codex and OpenCode requests using affected project profiles could fail with Invalid 'user': string too long.

After: Responses API requests carry a bounded, stable identifier before downstream namespacing. Personal profiles and non-Responses traffic remain unchanged.

Checklist

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