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/core-modules/billing/billing.exception.ts

@@ -30,6 +30,7 @@ export enum BillingExceptionCode {
   BILLING_PRICE_INVALID = 'BILLING_PRICE_INVALID',
   BILLING_SUBSCRIPTION_PHASE_NOT_FOUND = 'BILLING_SUBSCRIPTION_PHASE_NOT_FOUND',
   BILLING_TOO_MUCH_SUBSCRIPTIONS_FOUND = 'BILLING_TOO_MUCH_SUBSCRIPTIONS_FOUND',
+  BILLING_CREDITS_EXHAUSTED = 'BILLING_CREDITS_EXHAUSTED',
 }
 
 const billingExceptionUserFriendlyMessages: Record<
@@ -60,6 +61,7 @@ const billingExceptionUserFriendlyMessages: Record<
   [BillingExceptionCode.BILLING_PRICE_INVALID]: msg`Invalid price.`,
   [BillingExceptionCode.BILLING_SUBSCRIPTION_PHASE_NOT_FOUND]: msg`Subscription phase not found.`,
   [BillingExceptionCode.BILLING_TOO_MUCH_SUBSCRIPTIONS_FOUND]: msg`Multiple subscriptions found where one was expected.`,
+  [BillingExceptionCode.BILLING_CREDITS_EXHAUSTED]: msg`You have exhausted your credits. Please upgrade your plan to continue.`,
 };
 
 export class BillingException extends CustomException<BillingExceptionCode> {

packages/twenty-server/src/engine/core-modules/billing/filters/billing-api-exception.filter.ts

@@ -66,6 +66,12 @@ export class BillingRestApiExceptionFilter implements ExceptionFilter {
           response,
           400,
         );
+      case BillingExceptionCode.BILLING_CREDITS_EXHAUSTED:
+        return this.httpExceptionHandlerService.handleError(
+          exception,
+          response,
+          402,
+        );
       default:
         return this.httpExceptionHandlerService.handleError(
           exception,

packages/twenty-server/src/engine/core-modules/billing/services/billing.service.ts

@@ -3,12 +3,12 @@
 import { Injectable, Logger } from '@nestjs/common';
 import { InjectRepository } from '@nestjs/typeorm';
 
-import { isDefined } from 'class-validator';
-import { Repository } from 'typeorm';
+import { isDefined } from 'twenty-shared/utils';
+import { type Repository } from 'typeorm';
 
 import { BillingSubscriptionEntity } from 'src/engine/core-modules/billing/entities/billing-subscription.entity';
 import { type BillingEntitlementKey } from 'src/engine/core-modules/billing/enums/billing-entitlement-key.enum';
-import { BillingProductKey } from 'src/engine/core-modules/billing/enums/billing-product-key.enum';
+import { type BillingProductKey } from 'src/engine/core-modules/billing/enums/billing-product-key.enum';
 import { SubscriptionStatus } from 'src/engine/core-modules/billing/enums/billing-subscription-status.enum';
 import { BillingProductService } from 'src/engine/core-modules/billing/services/billing-product.service';
 import { BillingSubscriptionService } from 'src/engine/core-modules/billing/services/billing-subscription.service';