fix: fix intercept_warmup_requests config not being saved

Extract applyInterceptWarmup utility to unify all credential building
call sites:
- Fix upstream account creation missing intercept_warmup_requests write
- Fix apikey edit mode missing else-branch to clear the setting
- Add backend unit test for IsInterceptWarmupEnabled
- Add frontend unit test for credentialsBuilder

frontend/src/components/account/credentialsBuilder.ts

@@ -0,0 +1,11 @@
+export function applyInterceptWarmup(
+  credentials: Record<string, unknown>,
+  enabled: boolean,
+  mode: 'create' | 'edit'
+): void {
+  if (enabled) {
+    credentials.intercept_warmup_requests = true
+  } else if (mode === 'edit') {
+    delete credentials.intercept_warmup_requests
+  }
+}