feat(config): persist config.patch updates atomically

src/gateway/handlers/config.ts

@@ -4,6 +4,7 @@ import type { Config } from '../../config/index.js';
 
 export interface ConfigHandlerDeps {
   config: Config;
+  persistConfig?: (nextConfig: Config) => Promise<void> | void;
 }
 
 /**
@@ -140,6 +141,7 @@ export function createConfigHandlers(deps: ConfigHandlerDeps) {
 
       const applied: string[] = [];
       const rejected: string[] = [];
+      const draft = JSON.parse(JSON.stringify(deps.config)) as Config;
 
       for (const [key, value] of Object.entries(patches as Record<string, unknown>)) {
         const patcher = PATCHABLE_KEYS[key];
@@ -147,7 +149,7 @@ export function createConfigHandlers(deps: ConfigHandlerDeps) {
           rejected.push(key);
           continue;
         }
-        const ok = patcher(deps.config, value);
+        const ok = patcher(draft, value);
         if (ok) {
           applied.push(key);
         } else {
@@ -155,7 +157,30 @@ export function createConfigHandlers(deps: ConfigHandlerDeps) {
         }
       }
 
-      return makeResponse(request.id, { applied, rejected });
+      if (applied.length === 0) {
+        return makeResponse(request.id, { applied, rejected, persisted: false });
+      }
+
+      if (deps.persistConfig) {
+        try {
+          await deps.persistConfig(draft);
+        } catch (err) {
+          return makeResponse(request.id, {
+            applied: [],
+            rejected,
+            persisted: false,
+            persistError: err instanceof Error ? err.message : String(err),
+          });
+        }
+      }
+
+      // Update in-memory runtime config only after a successful persist (or when persistence is not configured).
+      for (const key of Object.keys(deps.config)) {
+        delete (deps.config as Record<string, unknown>)[key];
+      }
+      Object.assign(deps.config as Record<string, unknown>, draft as Record<string, unknown>);
+
+      return makeResponse(request.id, { applied, rejected, persisted: Boolean(deps.persistConfig) });
     },
   };
 }