fix: scope flash model names per auth type for Vertex AI and Gateway backends

[ibarsi]

Summary

Fixes #27759

The hasGemini35FlashGAAccess() method previously applied the same flash model names to all non-AI-Studio backends. This caused issues because different backends resolve model names differently:

• Vertex AI accepts gemini-3.5-flash but not the older gemini-3-flash alias, which results in a ModelNotFoundError
• Gateway / LOGIN_WITH_GOOGLE may not accept gemini-3.5-flash directly and needs the gemini-3-flash alias

Fix

Split the else branch into explicit cases per auth type:

Auth type	Preview model	Default model
USE_GEMINI (AI Studio)	gemini-3-flash-preview	gemini-3.5-flash
USE_VERTEX_AI	gemini-3.5-flash	gemini-3.5-flash
Other (Gateway)	gemini-3-flash	gemini-3-flash

Also adds afterEach cleanup in the test suite to reset flash model globals between tests, preventing cross-test contamination.

Test plan

• Added dedicated test for USE_VERTEX_AI auth type asserting both models are gemini-3.5-flash
• Updated existing non-USE_GEMINI test to use LOGIN_WITH_GOOGLE and assert gemini-3-flash for both models
• Added afterEach to reset model globals between tests
• All hasGemini35FlashGAAccess config tests pass

[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 a configuration issue where non-AI Studio backends were incorrectly defaulting to an unsupported model name when Gemini 3.5 Flash GA access was enabled. By standardizing the model resolution logic, the fix ensures consistent behavior across all authentication types and resolves deployment errors on Vertex AI.

Highlights

• Unified Model Configuration: Removed the auth-type conditional logic in hasGemini35FlashGAAccess to ensure all backends, including Vertex AI, consistently use the correct 'gemini-3.5-flash' model.
• Bug Fix: Resolved a 'ModelNotFoundError' occurring on Vertex AI by preventing the system from defaulting to the non-existent 'gemini-3-flash' model.
• Test Suite Update: Updated configuration tests to validate that 'USE_VERTEX_AI' correctly resolves to the expected Gemini 3.5 Flash model names.

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[github-actions]

📊 PR Size: size/S

• Lines changed: 30
• Additions: +22
• Deletions: -8
• Files changed: 2

[gemini-code-assist]

Code Review

This pull request simplifies the configuration of default Gemini Flash models when access is granted, removing the conditional check on authType so that gemini-3-flash-preview and gemini-3.5-flash are always set. The corresponding unit tests are updated to reflect this change. Feedback highlights that the test suite mutates module-level global variables without resetting them, which can lead to test flakiness, and suggests adding an afterEach block to restore the default values.