fix(gemini-vertex): add CLAUDE_CODE_USE_GEMINI_VERTEX to isUsing3PServices()

The exported isUsing3PServices() helper was missing the Gemini Vertex flag,
causing /login //logout to remain visible and auth status to report as
first-party when CLAUDE_CODE_USE_GEMINI_VERTEX=1. The local is3P check
inside isAnthropicAuthEnabled() was already correct; this aligns the
exported helper to match.

Adds a dedicated drift guard test targeting isUsing3PServices() directly,
since the existing test only covered the separate local is3P block.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/__tests__/bugfixes.test.ts

@@ -594,6 +594,17 @@ describe('Gemini Vertex provider drift guards', () => {
     expect(is3PSection).toContain('CLAUDE_CODE_USE_GEMINI_VERTEX')
   })
 
+  test('isUsing3PServices() includes Gemini Vertex flag', async () => {
+    const content = await file('utils/auth.ts').text()
+    const using3PSection = sectionBetween(
+      content,
+      'export function isUsing3PServices',
+      'function getConfiguredOtelHeadersHelper',
+    )
+
+    expect(using3PSection).toContain('CLAUDE_CODE_USE_GEMINI_VERTEX')
+  })
+
   test('API preconnect skips Gemini Vertex routing', async () => {
     const content = await file('utils/apiPreconnect.ts').text()
     const cloudSkipSection = sectionBetween(

src/utils/auth.ts

@@ -1759,6 +1759,7 @@ export function isUsing3PServices(): boolean {
     isEnvTruthy(process.env.CLAUDE_CODE_USE_FOUNDRY) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_OPENAI) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI) ||
+    isEnvTruthy(process.env.CLAUDE_CODE_USE_GEMINI_VERTEX) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_MISTRAL) ||
     isEnvTruthy(process.env.CLAUDE_CODE_USE_GITHUB)
   )