fix: address review feedback for Vercel AI Gateway PR

- Add --vercel-ai-gateway CLI option to install command
- Fix librarian/explore priority: native providers (ZAI, Copilot) now take precedence over Vercel gateway
- Add hasVercelAiGateway to installer no-provider warning conditions
- Add hasVercelAiGateway: false to all test file InstallConfig fixtures
- Fix test expectations for model ID format (claude-opus-4.6 vs claude-opus-4-6)

src/cli/cli-installer.test.ts

@@ -37,6 +37,7 @@ describe("runCliInstaller", () => {
         hasZaiCodingPlan: false,
         hasKimiForCoding: false,
         hasOpencodeGo: false,
+        hasVercelAiGateway: false,
       }),
       spyOn(configManager, "isOpenCodeInstalled").mockResolvedValue(true),
       spyOn(configManager, "getOpenCodeVersion").mockResolvedValue("1.3.9"),
@@ -83,6 +84,7 @@ describe("runCliInstaller", () => {
         hasZaiCodingPlan: false,
         hasKimiForCoding: false,
         hasOpencodeGo: false,
+        hasVercelAiGateway: false,
       }),
       spyOn(configManager, "isOpenCodeInstalled").mockResolvedValue(true),
       spyOn(configManager, "getOpenCodeVersion").mockResolvedValue("1.4.0"),