Linking GCP billing account does not upgrade Gemini API from free-tier quotas

### Problem

After linking a GCP billing account (with active credits) to a project, the Gemini API continues to enforce **free-tier quotas** (`generate_content_free_tier_requests`, limit: 20 req/day/model). There is no indication in any UI that billing linkage didn't actually upgrade the API tier.

### Steps to Reproduce

1. Create a GCP project
2. Link a billing account with active credits
3. Enable the Generative Language API
4. Create an API key from the project credentials page
5. Make API calls to `gemini-3-flash-preview` via `@google/genai` SDK
6. After hitting 20 requests → still get free-tier quota error:

```
Quota exceeded for metric: generativelanguage.googleapis.com/generate_content_free_tier_requests
quotaId: GenerateRequestsPerDayPerProjectPerModel-FreeTier
```

### Expected Behavior

Linking a billing account with active credits should upgrade the project from free-tier to pay-as-you-go quotas, or at minimum **clearly indicate** that additional steps are required to enable paid usage.

### Actual Behavior

- GCP Console shows billing is linked
- API quota page still shows **free tier** quotas at 100% usage
- No error, warning, or guidance anywhere that billing isn't effective for Gemini API keys
- Significant time wasted debugging what appeared to be a correctly configured setup

### Impact

Developers who follow the standard GCP flow (create project → link billing → create key) reasonably expect paid quotas to apply. The silent failure to upgrade tiers, combined with zero UI feedback, creates a frustrating developer experience.