fix: clear error messages for missing API keys on provider switch

Previously, switching to zhipuai/openrouter/xai via /model would throw a
confusing 'OPENAI_API_KEY missing' error from the OpenAI SDK. Now
createClientFromConfig validates API keys before constructing the client,
throwing errors that name the correct env var (e.g. ZHIPUAI_API_KEY).

Also fixes the misleading 'as anthropic' type cast in the /model handler
to validate against MODEL_PROVIDERS and use the ModelProvider type.

src/frontends/tui/minimal.ts

@@ -5,7 +5,8 @@ import type { ModelRouter, ModelTier } from '../../models/router.js';
 import type { NativeAgent } from '../../backends/native/agent.js';
 import { parseCommand, getHelpText, resolveModelAlias, getCommandCompletions, getCommandTooltip, type Command } from './commands.js';
 import { renderMarkdown } from './markdown.js';
-import type { ModelConfig } from '../../config/schema.js';
+import type { ModelConfig, ModelProvider } from '../../config/schema.js';
+import { MODEL_PROVIDERS } from '../../config/schema.js';
 import { OllamaClient, LlamaCppClient } from '../../models/index.js';
 import { createClientFromConfig } from '../../daemon/index.js';
 import { loginGitHub } from '../../auth/index.js';
@@ -232,8 +233,13 @@ export class MinimalTui {
       const provider = providerModel.slice(0, slashIdx);
       const model = providerModel.slice(slashIdx + 1);
 
+      if (!MODEL_PROVIDERS.includes(provider as ModelProvider)) {
+        console.log(`${colors.gray}Unknown provider "${provider}". Known providers: ${MODEL_PROVIDERS.join(', ')}${colors.reset}\n`);
+        return;
+      }
+
       try {
-        const client = createClientFromConfig({ provider: provider as 'anthropic', model });
+        const client = createClientFromConfig({ provider: provider as ModelProvider, model });
         router.setClient(tier, client, providerModel);
         console.log(`${colors.gray}Set ${tier} to:${colors.reset} ${providerModel}\n`);
       } catch (error) {