feat(proxy): session-based prompt_cache_key routing for Codex Chat bridge

Codex always sends prompt_cache_key in its Responses requests, but the
Responses -> Chat Completions conversion dropped it, breaking session
cache affinity on upstreams that route by key (e.g. Kimi Coding).

- Re-inject prompt_cache_key after conversion in the forwarder: an
  explicit client key wins, otherwise a client-provided session ID;
  generated per-request UUIDs are never sent upstream.
- Provider-aware gating: "auto" enables only known-compatible upstreams
  (api.openai.com, api.kimi.com/coding) because strict gateways reject
  unknown fields with HTTP 400 (e.g. Fireworks); an advanced
  Auto/Enabled/Disabled override is available on the Codex form in all
  four locales.
- Kimi For Coding preset opts in explicitly.

src-tauri/src/proxy/providers/mod.rs

@@ -52,8 +52,9 @@ pub use claude::{
 pub use codex::CodexAdapter;
 pub use codex::{
     apply_codex_chat_upstream_model, apply_codex_upstream_model, codex_provider_upstream_model,
-    resolve_codex_catalog_tool_profile, resolve_codex_chat_reasoning_config,
-    should_convert_codex_responses_to_anthropic, should_convert_codex_responses_to_chat,
+    inject_codex_chat_prompt_cache_key, resolve_codex_catalog_tool_profile,
+    resolve_codex_chat_reasoning_config, should_convert_codex_responses_to_anthropic,
+    should_convert_codex_responses_to_chat,
 };
 pub use gemini::GeminiAdapter;