fix(gateway): block llmgateway provider in credits mode

[steebchen]

Summary

• Fix confusing error "No API key set in environment for provider: llmgateway" when using the "custom" model in credits mode
• The "custom" model uses providerId "llmgateway", but the credits mode check only blocked provider === "custom"
• Now also blocks provider === "llmgateway" in credits mode with the correct error message

Test plan

• pnpm build passes
• pnpm test:unit passes (390 tests)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Extended credits-mode restrictions: The llmgateway provider is now blocked in credits mode, aligning with existing custom provider limitations. Requests using these providers in credits mode will be rejected.

[coderabbitai]

Walkthrough

This PR expands the credits-mode restriction to block both "custom" and "llmgateway" providers from being used in requests. Two conditional checks in the chat endpoint were updated to reject requests with either provider type when the project operates in credits mode.

Changes

Cohort / File(s)	Summary
Credits Mode Provider Restriction apps/gateway/src/chat/chat.ts	Expanded conditional checks from blocking only "custom" provider to also blocking "llmgateway" provider in credits mode. The condition was modified in two locations to reject both provider types with a 400 error.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• fix(chat): update provider handling and error messages #314: Directly related changes to provider handling for the "llmgateway" provider in credits-mode restrictions within the same chat.ts file.

Suggested reviewers

• smakosh

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: blocking the llmgateway provider in credits mode, which directly addresses the confusing error message issue.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch steebchen/fix-llmgateway-provider

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