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/config/codexProviderPresets.ts

@@ -6,6 +6,7 @@ import type {
   CodexApiFormat,
   CodexCatalogModel,
   CodexChatReasoning,
+  PromptCacheRoutingMode,
 } from "../types";
 import type { PresetTheme } from "./claudeProviderPresets";
 
@@ -36,6 +37,8 @@ export interface CodexProviderPreset {
   modelCatalog?: CodexCatalogModel[];
   // Codex Responses -> Chat Completions reasoning capability defaults
   codexChatReasoning?: CodexChatReasoning;
+  // Session-based prompt-cache routing override for Chat Completions upstreams
+  promptCacheRouting?: PromptCacheRoutingMode;
 }
 
 /**
@@ -604,6 +607,7 @@ requires_openai_auth = true`,
     ),
     endpointCandidates: ["https://api.kimi.com/coding/v1"],
     apiFormat: "openai_chat",
+    promptCacheRouting: "enabled",
     modelCatalog: modelCatalog([
       {
         model: "kimi-for-coding",