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/__tests__/credentialsBuilder.spec.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'vitest'
+import { applyInterceptWarmup } from '../credentialsBuilder'
+
+describe('applyInterceptWarmup', () => {
+  it('create + enabled=true: should set intercept_warmup_requests to true', () => {
+    const creds: Record<string, unknown> = { access_token: 'tok' }
+    applyInterceptWarmup(creds, true, 'create')
+    expect(creds.intercept_warmup_requests).toBe(true)
+  })
+
+  it('create + enabled=false: should not add the field', () => {
+    const creds: Record<string, unknown> = { access_token: 'tok' }
+    applyInterceptWarmup(creds, false, 'create')
+    expect('intercept_warmup_requests' in creds).toBe(false)
+  })
+
+  it('edit + enabled=true: should set intercept_warmup_requests to true', () => {
+    const creds: Record<string, unknown> = { api_key: 'sk' }
+    applyInterceptWarmup(creds, true, 'edit')
+    expect(creds.intercept_warmup_requests).toBe(true)
+  })
+
+  it('edit + enabled=false + field exists: should delete the field', () => {
+    const creds: Record<string, unknown> = { api_key: 'sk', intercept_warmup_requests: true }
+    applyInterceptWarmup(creds, false, 'edit')
+    expect('intercept_warmup_requests' in creds).toBe(false)
+  })
+
+  it('edit + enabled=false + field absent: should not throw', () => {
+    const creds: Record<string, unknown> = { api_key: 'sk' }
+    applyInterceptWarmup(creds, false, 'edit')
+    expect('intercept_warmup_requests' in creds).toBe(false)
+  })
+
+  it('should not affect other fields', () => {
+    const creds: Record<string, unknown> = {
+      api_key: 'sk',
+      base_url: 'url',
+      intercept_warmup_requests: true
+    }
+    applyInterceptWarmup(creds, false, 'edit')
+    expect(creds.api_key).toBe('sk')
+    expect(creds.base_url).toBe('url')
+    expect('intercept_warmup_requests' in creds).toBe(false)
+  })
+})