fix(cli): add GATEWAY auth type to validateAuthMethod

[tmuskal]

Summary

AuthType.GATEWAY was added in b705505 (PR #27021) to support custom base URL routing via GOOGLE_GEMINI_BASE_URL, but validateAuthMethod() in packages/cli/src/config/auth.ts was not updated to handle this new auth type.

When GOOGLE_GEMINI_BASE_URL is set (with GOOGLE_GENAI_USE_VERTEXAI=false), getAuthTypeFromEnv() correctly returns AuthType.GATEWAY, but validateAuthMethod() falls through all existing handlers (LOGIN_WITH_GOOGLE, COMPUTE_ADC, USE_GEMINI, USE_VERTEX_AI) and returns "Invalid auth method selected." (exit code 41).

Root cause

validateAuthMethod() handles four auth types but was missing GATEWAY:

// Before (missing GATEWAY)
if (authMethod === AuthType.LOGIN_WITH_GOOGLE || authMethod === AuthType.COMPUTE_ADC) return null;
if (authMethod === AuthType.USE_GEMINI) { /* validate GEMINI_API_KEY */ }
if (authMethod === AuthType.USE_VERTEX_AI) { /* validate project/location */ }
return 'Invalid auth method selected.'; // ← GATEWAY falls through here

Fix

Added AuthType.GATEWAY handler that returns null (valid, no validation needed). Gateway auth delegates authentication to the gateway itself — API keys are optional, handled downstream in createContentGeneratorConfig which falls back to GEMINI_API_KEY or empty string.

Changes

• packages/cli/src/config/auth.ts — add GATEWAY case to validateAuthMethod()
• packages/cli/src/config/auth.test.ts — add test case for GATEWAY auth type

Test plan

• npx vitest run packages/cli/src/config/auth.test.ts — 9/9 tests pass (including new GATEWAY test)
• npx vitest run packages/core/src/core/contentGenerator.test.ts — 47/47 tests pass (no regressions)
• Verified git diff contains only the two intended file changes

Fixes #27550

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where the CLI failed to recognize the GATEWAY authentication method, causing it to incorrectly report an invalid auth method error. By updating the validation logic to include this type, the system now properly delegates authentication to the gateway as intended.

Highlights

• Authentication Validation: Updated the validateAuthMethod function to correctly recognize and validate the GATEWAY authentication type.
• Test Coverage: Added a new test case in auth.test.ts to ensure that GATEWAY authentication is handled correctly and returns no validation errors.

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[gemini-code-assist]

Code Review

This pull request adds support for the GATEWAY authentication type in the CLI configuration. Specifically, it updates validateAuthMethod in packages/cli/src/config/auth.ts to return null (indicating a valid authentication method) when the auth method is AuthType.GATEWAY, and adds a corresponding unit test in packages/cli/src/config/auth.test.ts to verify this behavior. I have no feedback to provide.