feat: add AI chat error handling for billing and API key errors (#16797)

## Summary

This PR adds user-friendly error handling for AI chat features,
specifically for **billing credits exhausted** and **API key not
configured** errors.

## Changes

### Backend
- Added `BILLING_CREDITS_EXHAUSTED` exception code with 402 status
- Added `API_KEY_NOT_CONFIGURED` exception code with 503 status
- Added billing check before AI chat streaming in
`agent-chat.controller.ts`
- Added error code to HTTP exception response body for frontend error
type detection
- Created `AgentRestApiExceptionFilter` for agent-specific errors

### Frontend
- Created `AIChatBanner` - reusable banner component for error/warning
messages
- Created `AIChatCreditsExhaustedMessage` - shows upgrade prompts based
on user permissions
- Created `AIChatApiKeyNotConfiguredMessage` - shows configuration
guidance with docs link
- Created `AIChatErrorRenderer` - encapsulates error type switching
logic (fixes nested ternary)
- Created `AIChatStandaloneError` - displays errors when there are no
messages
- Split `aiChatErrorUtils.ts` into separate files (1 export per file):
  - `AIChatErrorCode.ts`
  - `extractErrorCode.ts`
  - `isAIChatErrorOfType.ts`
  - `isBillingCreditsExhaustedError.ts`
  - `isApiKeyNotConfiguredError.ts`
- Added comprehensive test coverage (27 tests)

### Other
- Updated trial period banner messaging

## Testing
- All lint checks pass
- All 27 new tests pass
- TypeScript typecheck passes

packages/twenty-server/src/engine/metadata-modules/ai/ai-chat/controllers/agent-chat.controller.ts

@@ -7,26 +7,48 @@ import {
   UseGuards,
 } from '@nestjs/common';
 
-import { Response } from 'express';
-import { type ExtendedUIMessage } from 'twenty-shared/ai';
 import { PermissionFlagType } from 'twenty-shared/constants';
 
+import type { Response } from 'express';
+import type { ExtendedUIMessage } from 'twenty-shared/ai';
+
 import { RestApiExceptionFilter } from 'src/engine/api/rest/rest-api-exception.filter';
-import { type WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
+import {
+  BillingException,
+  BillingExceptionCode,
+} from 'src/engine/core-modules/billing/billing.exception';
+import { BillingProductKey } from 'src/engine/core-modules/billing/enums/billing-product-key.enum';
+import { BillingRestApiExceptionFilter } from 'src/engine/core-modules/billing/filters/billing-api-exception.filter';
+import { BillingService } from 'src/engine/core-modules/billing/services/billing.service';
+import { TwentyConfigService } from 'src/engine/core-modules/twenty-config/twenty-config.service';
+import type { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
 import { AuthUserWorkspaceId } from 'src/engine/decorators/auth/auth-user-workspace-id.decorator';
 import { AuthWorkspace } from 'src/engine/decorators/auth/auth-workspace.decorator';
 import { JwtAuthGuard } from 'src/engine/guards/jwt-auth.guard';
 import { SettingsPermissionGuard } from 'src/engine/guards/settings-permission.guard';
 import { WorkspaceAuthGuard } from 'src/engine/guards/workspace-auth.guard';
-import { type BrowsingContextType } from 'src/engine/metadata-modules/ai/ai-agent/types/browsingContext.type';
+import {
+  AgentException,
+  AgentExceptionCode,
+} from 'src/engine/metadata-modules/ai/ai-agent/agent.exception';
+import { AgentRestApiExceptionFilter } from 'src/engine/metadata-modules/ai/ai-agent/filters/agent-api-exception.filter';
+import type { BrowsingContextType } from 'src/engine/metadata-modules/ai/ai-agent/types/browsingContext.type';
 import { AgentChatStreamingService } from 'src/engine/metadata-modules/ai/ai-chat/services/agent-chat-streaming.service';
+import { AiModelRegistryService } from 'src/engine/metadata-modules/ai/ai-models/services/ai-model-registry.service';
 
 @Controller('rest/agent-chat')
 @UseGuards(JwtAuthGuard, WorkspaceAuthGuard)
-@UseFilters(RestApiExceptionFilter)
+@UseFilters(
+  AgentRestApiExceptionFilter,
+  BillingRestApiExceptionFilter,
+  RestApiExceptionFilter,
+)
 export class AgentChatController {
   constructor(
     private readonly agentStreamingService: AgentChatStreamingService,
+    private readonly billingService: BillingService,
+    private readonly twentyConfigService: TwentyConfigService,
+    private readonly aiModelRegistryService: AiModelRegistryService,
   ) {}
 
   @Post('stream')
@@ -42,6 +64,29 @@ export class AgentChatController {
     @AuthWorkspace() workspace: WorkspaceEntity,
     @Res() response: Response,
   ) {
+    const availableModels = this.aiModelRegistryService.getAvailableModels();
+
+    if (availableModels.length === 0) {
+      throw new AgentException(
+        'No AI models are available. Please configure at least one AI provider API key (OPENAI_API_KEY, ANTHROPIC_API_KEY, or XAI_API_KEY).',
+        AgentExceptionCode.API_KEY_NOT_CONFIGURED,
+      );
+    }
+
+    if (this.twentyConfigService.get('IS_BILLING_ENABLED')) {
+      const canBill = await this.billingService.canBillMeteredProduct(
+        workspace.id,
+        BillingProductKey.WORKFLOW_NODE_EXECUTION,
+      );
+
+      if (!canBill) {
+        throw new BillingException(
+          'Credits exhausted',
+          BillingExceptionCode.BILLING_CREDITS_EXHAUSTED,
+        );
+      }
+    }
+
     this.agentStreamingService.streamAgentChat({
       threadId: body.threadId,
       messages: body.messages,