Align GraphQL error handling for billing and AI chat (#19690)

## What changed

This refactor fixes AI chat error surfacing by aligning both the backend
and frontend with the existing GraphQL error architecture instead of
adding AI-local error translation.

On the backend:
- add a dedicated GraphQL billing exception path
- register billing GraphQL handling globally for GraphQL requests
- reuse the existing AI GraphQL interceptor path for agent/chat
exceptions
- keep billing status classification shared between REST and GraphQL
- remove the earlier attempt to preserve `CustomException` metadata in
the global GraphQL fallback

On the frontend:
- keep the original Apollo GraphQL error object in AI chat state
- reuse shared Apollo/GraphQL helpers for user-facing messages and
error-type checks
- delete AI-specific error extraction helpers that duplicated generic
GraphQL parsing
- replace a few direct `extensions.subCode` call sites with a shared
predicate

## Why it changed

The original bug was that `BillingException` and AI exceptions thrown
from chat were not being translated into GraphQL errors with the
expected `extensions.subCode` and `extensions.userFriendlyMessage`, so
the AI chat UI had nothing structured to inspect.

An intermediate fix worked mechanically but pushed `CustomException`
handling into the global GraphQL fallback, which blurred the intended
layering. This PR moves the behavior back to explicit GraphQL edges.

## Root cause

`AgentChatResolver` could throw `BillingException` and `AgentException`,
but:
- billing had a REST exception filter and no shared GraphQL equivalent
- AI chat was not consistently using the same GraphQL exception
translation path as the sibling AI resolver
- the frontend chat UI had drifted into AI-specific error parsing
instead of consuming the same structured Apollo errors as the rest of
the app

## Impact

- `BILLING_CREDITS_EXHAUSTED` is now preserved through GraphQL and can
render the existing credits-exhausted UI in chat
- `API_KEY_NOT_CONFIGURED` is preserved through the AI GraphQL path
- AI chat now follows the same general GraphQL error consumption pattern
as the rest of the frontend
- billing GraphQL handling is less dependent on individual resolver
authors remembering to add a filter

## Validation

- `yarn jest --config packages/twenty-server/jest.config.mjs
packages/twenty-server/src/engine/core-modules/billing/utils/__tests__/billing-graphql-api-exception-handler.util.spec.ts
packages/twenty-server/src/engine/metadata-modules/ai/ai-agent/utils/__tests__/agent-graphql-api-exception-handler.util.spec.ts`
- `yarn jest --config packages/twenty-front/jest.config.mjs
packages/twenty-front/src/utils/__tests__/is-graphql-error-of-type.util.test.ts`
- `npx oxlint --type-aware ...` on touched backend/frontend files
- `npx prettier --check ...` on touched backend/frontend files

## Follow-up ideas

- consolidate frontend GraphQL error helpers further so more existing
direct `extensions.subCode` checks move to shared utilities
- consider whether common GraphQL exception filter registration should
live in a more explicit GraphQL-specific module instead of
`CoreEngineModule`
- add an end-to-end test for a real `sendChatMessage` GraphQL failure
path in AI chat

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/twenty-front/src/modules/auth/hooks/useAuth.ts

@@ -3,7 +3,6 @@ import {
   useLazyQuery,
   useMutation,
 } from '@apollo/client/react';
-import { CombinedGraphQLErrors } from '@apollo/client/errors';
 import { useCallback } from 'react';
 import { AppPath } from 'twenty-shared/types';
 
@@ -67,6 +66,7 @@ import { useNavigate, useSearchParams } from 'react-router-dom';
 import { SOURCE_LOCALE } from 'twenty-shared/translations';
 import { isDefined } from 'twenty-shared/utils';
 import { getWorkspaceUrl } from '~/utils/getWorkspaceUrl';
+import { isGraphqlErrorOfType } from '~/utils/is-graphql-error-of-type.util';
 import { useStore } from 'jotai';
 
 export const useAuth = () => {
@@ -208,11 +208,7 @@ export const useAuth = () => {
 
         return getLoginTokenResult.data.getLoginTokenFromCredentials;
       } catch (error) {
-        // TODO: Get intellisense for graphql error extensions code (codegen?)
-        if (
-          CombinedGraphQLErrors.is(error) &&
-          error.errors[0]?.extensions?.subCode === 'EMAIL_NOT_VERIFIED'
-        ) {
+        if (isGraphqlErrorOfType(error, 'EMAIL_NOT_VERIFIED')) {
           setSearchParams({ email });
           setSignInUpStep(SignInUpStep.EmailVerification);
           throw error;
@@ -334,9 +330,10 @@ export const useAuth = () => {
         );
       } catch (error) {
         if (
-          CombinedGraphQLErrors.is(error) &&
-          error.errors[0]?.extensions?.subCode ===
-            'TWO_FACTOR_AUTHENTICATION_PROVISION_REQUIRED'
+          isGraphqlErrorOfType(
+            error,
+            'TWO_FACTOR_AUTHENTICATION_PROVISION_REQUIRED',
+          )
         ) {
           handleSetLoginToken(loginToken);
           navigate(AppPath.SignInUp);
@@ -344,9 +341,10 @@ export const useAuth = () => {
         }
 
         if (
-          CombinedGraphQLErrors.is(error) &&
-          error.errors[0]?.extensions?.subCode ===
-            'TWO_FACTOR_AUTHENTICATION_VERIFICATION_REQUIRED'
+          isGraphqlErrorOfType(
+            error,
+            'TWO_FACTOR_AUTHENTICATION_VERIFICATION_REQUIRED',
+          )
         ) {
           handleSetLoginToken(loginToken);
           navigate(AppPath.SignInUp);
@@ -399,10 +397,7 @@ export const useAuth = () => {
           setSignInUpStep(SignInUpStep.WorkspaceSelection);
         },
         onError: (error) => {
-          if (
-            CombinedGraphQLErrors.is(error) &&
-            error.errors[0]?.extensions?.subCode === 'EMAIL_NOT_VERIFIED'
-          ) {
+          if (isGraphqlErrorOfType(error, 'EMAIL_NOT_VERIFIED')) {
             setSearchParams({ email });
             setSignInUpStep(SignInUpStep.EmailVerification);
             throw error;