support official DeepSeek Responses

[Rat0323]

Summary

• switch the built-in DeepSeek preset from Chat Completions to DeepSeek's official Responses API
• preserve both deepseek-v4-flash and deepseek-v4-pro; upstream remains the source of truth for availability
• generate the model metadata required by Codex for the official DeepSeek Responses models
• disable Code Mode only for Responses profiles whose effective endpoint is deepseek.com or a subdomain
• preserve Unified Exec, Chat Completions, third-party relay behavior, and the existing relay profile lifecycle
• retain the independent .gitattributes EOL normalization fix used by upstream asset checks

Scope

The provider presets are convenience templates that populate profile fields; Codex++ does not persist a preset identity.

This compatibility therefore applies at the profile level. It is enabled when:

1. protocol = responses
2. the effective endpoint host is deepseek.com or one of its subdomains

A profile created manually with the official DeepSeek Responses endpoint receives the same compatibility handling. Changing the preset URL to a third-party relay leaves that relay's existing behavior unchanged.

This PR does not infer provider capabilities from model names and does not add a generic DeepSeek compatibility mode. Third-party relays may expose DeepSeek, GLM, Doubao, GPT, or mixed catalogs with different tool behavior.

Root cause

Codex Code Mode publishes a Responses custom tool named exec. DeepSeek's official Responses endpoint rejects it with Unsupported custom tool: 'exec'.

For official DeepSeek Responses profiles, Codex++ writes the effective live configuration with:

[features]
code_mode_only = false

[features.code_mode]
enabled = false

Unified Exec is a separate path that publishes the function tools exec_command and write_stdin, so it remains unchanged.

Chat Completions also remains available through Codex++'s existing protocol proxy. It does not use this Responses-specific workaround.

The existing active-profile save flow is preserved. Its backend config write now applies the same official-endpoint compatibility transform, so saving an active official DeepSeek Responses profile cannot bypass the adapter.

Validation

• npm run check
• npm test (37 passed)
• cargo fmt --all -- --check
• cargo test -p codex-plus-manager --lib --locked -- --test-threads=1 (43 passed)
• cargo test -p codex-plus-core --lib --locked -- --test-threads=1 (208 passed)
• cargo test -p codex-plus-core --test relay_config --test relay_switch --test model_suffix --locked -- --test-threads=1 (132 passed)
• git diff --check

Regression coverage includes manually created official DeepSeek Responses profiles, official-login profiles mixed with an API key, third-party Responses profiles, and official DeepSeek Chat Completions profiles.