[Bug]: Chat fails with "Unknown provider: custom-xxx" for custom providers

[nkmohit]

Bug Description

When a custom provider is configured via the Settings UI (e.g. OpenRouter or any self-hosted endpoint), sending a message, starting a discussion, or resuming a session in the chat panel fails with a 500 error.

Steps to Reproduce

1. Configure a custom provider in Settings (base URL + API key + model)
2. Generate a classroom with that provider
3. Send a message in the chat panel

Expected Behavior

Chat responds normally using the configured custom provider.

Actual Behavior

500 error: Unknown provider: custom-1773899122192. Please provide providerType.

Deployment Method

Local development (npm run dev / pnpm dev / yarn dev)

Browser

Brave

Operating System

macOS 26.2

Relevant Logs / Screenshots

[2026-03-19T08:37:39.497Z] [ERROR] [ChatSessions] [ChatArea] Error: Error: API error: 500 - {"success":false,"errorCode":"INTERNAL_ERROR","error":"Unknown provider: custom-1773899122192. Please provide providerType."}
    at useChatSessions.useCallback[runAgentLoop] (http://localhost:3000/_next/static/chunks/Developer_OpenMAIC_components_chat_a0ed9d01._.js:871:27)
    at async useChatSessions.useCallback[sendMessage] [as sendMessage] (http://localhost:3000/_next/static/chunks/Developer_OpenMAIC_components_chat_a0ed9d01._.js:1456:17)

Additional Context

Fix available in PR #114. Root cause: /api/chat used an older manual provider resolution path that didn't forward providerType, unlike all other API routes which already used resolveModel().

[wyuc]

Fixed by #114 which forwards providerType through the chat request path and uses the shared resolveModel() helper.