fix: show clear error when Cloudflare provider env vars are missing

[mchenco]

Issue for this PR

Fixes #20398

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When users connect cloudflare-workers-ai or cloudflare-ai-gateway via /connect, the API key is saved in auth. But the custom loaders also need CLOUDFLARE_ACCOUNT_ID (and CLOUDFLARE_GATEWAY_ID for gateway) as env vars.

When those env vars are missing, the loaders return { autoload: false } without a getModel. Since the provider is already in the providers map from auth, the loader result is still processed (result.autoload || providers[providerID]), but no model loader gets registered. When the user sends a message, the fallback calls sdk.languageModel(model.api.id) which crashes with sdk.languageModel is not a function.

The fix adds a getModel to the early returns that throws a clear error naming the missing env var with the export command, e.g.:

CLOUDFLARE_ACCOUNT_ID is missing. Set it with: export CLOUDFLARE_ACCOUNT_ID=<your-account-id>

How did you verify your code works?

• All 70 existing provider tests pass
• Ran a verification script that simulates /connect auth without CLOUDFLARE_ACCOUNT_ID, confirms the provider loads from auth, and verifies getLanguage throws the clear error instead of crashing

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[elithrar]

LGTM from our/the Cloudflare side. Makes the missing env vars more explicit.