fix DeepSeek Responses tool compatibility

[Rat0323]

Summary

• configure the built-in DeepSeek preset to use DeepSeek's official Responses API integration while keeping the same https://api.deepseek.com/ base URL
• generate a DeepSeek-specific model catalog with the official tool and context metadata
• disable Unified Exec and both Code Mode selectors only for DeepSeek Responses profiles
• preserve AgentRouter, other Responses providers, and DeepSeek Chat Completions behavior

Root cause

DeepSeek Responses currently accepts the apply_patch custom tool, but Codex++ could also expose custom tools named exec through Unified Exec or Code Mode. Disabling only Unified Exec was insufficient because features.code_mode and features.code_mode_only independently select Codex Code Mode, whose public freeform tool is also named exec.

The compatibility pass now writes:

[features]
unified_exec = false
code_mode_only = false

[features.code_mode]
enabled = false

Existing nested Code Mode settings such as direct_only_tool_namespaces are preserved.

Validation

• cargo test -p codex-plus-core --test relay_config (110 passed)
• cargo test -p codex-plus-core --test model_suffix (15 passed)
• npm test in apps/codex-plus-manager (37 passed)
• manually verified with DeepSeek in Codex task 019fcb0f-f11f-7d12-a026-072b8304d297: Shell and Node REPL tool calls completed successfully without Unsupported custom tool: 'exec'