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/components/providers/forms/ProviderForm.tsx

@@ -21,6 +21,7 @@ import type {
   CodexApiFormat,
   CodexCatalogModel,
   CodexChatReasoning,
+  PromptCacheRoutingMode,
   ClaudeApiKeyField,
 } from "@/types";
 import {
@@ -357,6 +358,7 @@ function ProviderFormFull({
       ),
     });
     setCodexChatReasoning(initialData?.meta?.codexChatReasoning ?? {});
+    setPromptCacheRouting(initialData?.meta?.promptCacheRouting ?? "auto");
     setCustomUserAgent(initialData?.meta?.customUserAgent ?? "");
     setLocalProxyHeadersOverride(
       formatRequestOverrideObject(
@@ -529,6 +531,10 @@ function ProviderFormFull({
     useState<CodexChatReasoning>(
       () => initialData?.meta?.codexChatReasoning ?? {},
     );
+  const [promptCacheRouting, setPromptCacheRouting] =
+    useState<PromptCacheRoutingMode>(
+      () => initialData?.meta?.promptCacheRouting ?? "auto",
+    );
   const [customUserAgent, setCustomUserAgent] = useState<string>(
     () => initialData?.meta?.customUserAgent ?? "",
   );
@@ -632,6 +638,7 @@ function ProviderFormFull({
       const template = getCodexCustomTemplate();
       resetCodexConfig(template.auth, template.config);
       setCodexChatReasoning({});
+      setPromptCacheRouting("auto");
     }
   }, [appId, initialData, selectedPresetId, resetCodexConfig]);
 
@@ -1485,6 +1492,13 @@ function ProviderFormFull({
         localCodexApiFormat === "openai_chat"
           ? normalizeCodexChatReasoningForSave(codexChatReasoning)
           : undefined,
+      promptCacheRouting:
+        appId === "codex" &&
+        category !== "official" &&
+        localCodexApiFormat === "openai_chat" &&
+        promptCacheRouting !== "auto"
+          ? promptCacheRouting
+          : undefined,
       customUserAgent:
         (appId === "claude" || appId === "codex") && category !== "official"
           ? customUserAgent.trim() || undefined
@@ -1651,6 +1665,7 @@ function ProviderFormFull({
         const template = getCodexCustomTemplate();
         resetCodexConfig(template.auth, template.config);
         setCodexChatReasoning({});
+        setPromptCacheRouting("auto");
         setLocalCodexApiFormat(
           codexApiFormatFromWireApi(extractCodexWireApi(template.config)) ??
             "openai_responses",
@@ -1692,6 +1707,7 @@ function ProviderFormFull({
 
       resetCodexConfig(auth, config, preset.modelCatalog ?? []);
       setCodexChatReasoning(preset.codexChatReasoning ?? {});
+      setPromptCacheRouting(preset.promptCacheRouting ?? "auto");
       setLocalCodexApiFormat(
         preset.apiFormat ??
           codexApiFormatFromWireApi(extractCodexWireApi(config)) ??
@@ -2182,6 +2198,8 @@ function ProviderFormFull({
               onMaxOutputTokensChange={setLocalCodexMaxOutputTokens}
               codexChatReasoning={codexChatReasoning}
               onCodexChatReasoningChange={setCodexChatReasoning}
+              promptCacheRouting={promptCacheRouting}
+              onPromptCacheRoutingChange={setPromptCacheRouting}
               catalogModels={codexCatalogModels}
               onCatalogModelsChange={setCodexCatalogModels}
               speedTestEndpoints={speedTestEndpoints}