refactor(server): rename Agent exception to Ai; add THREAD_NOT_FOUND / MESSAGE_NOT_FOUND codes (fixes 500s) (#19831)

## Summary

- The exception class under `ai-agent/` was serving every AI surface
(agent, chat, role, models, generate-text), so `Agent` was a misnomer.
Promoted to the `ai/` namespace; renamed `AgentException` →
`AiException`, `AgentExceptionCode` → `AiExceptionCode`, and related
interceptor / filter / handler / file names accordingly.
- Split the single `AGENT_NOT_FOUND` code into entity-specific codes.
Chat-thread lookups no longer reuse the agent identifier.
- **Fixes Sentry 500s on `GetChatMessages` / `chatThread`.** Every
"Thread not found" and "Queued message not found" throw site in ai-chat
was previously wired to `AGENT_EXECUTION_FAILED`, which maps to
`InternalServerError` (HTTP 500). They now use `THREAD_NOT_FOUND` /
`MESSAGE_NOT_FOUND`, both of which map to `NotFoundError` (HTTP 404) in
the GraphQL and REST handlers.

The underlying cause of *why* clients are asking for threads that no
longer resolve for them — per-user chat-thread create events being
broadcast workspace-wide — is addressed separately in a follow-up PR.

### Code map

- Added: `ai/ai.exception.ts`,
`ai/utils/ai-graphql-api-exception-handler.util.ts` (+ spec with new
THREAD/MESSAGE cases),
`ai/interceptors/ai-graphql-api-exception.interceptor.ts`,
`ai/filters/ai-api-exception.filter.ts`
- Deleted: `ai/ai-agent/agent.exception.ts`,
`ai/ai-agent/utils/agent-graphql-api-exception-handler.util.ts` (+
spec),
`ai/ai-agent/interceptors/agent-graphql-api-exception.interceptor.ts`,
`ai/ai-agent/filters/agent-api-exception.filter.ts`
- Updated: 21 call sites across ai-agent, ai-agent-execution,
ai-agent-role, ai-chat, ai-generate-text, ai-models, role, and
workspace-migration validators.

## Test plan

- [x] `npx nx typecheck twenty-server`
- [x] `npx jest ai-graphql-api-exception-handler` (3/3 including new
THREAD_NOT_FOUND and MESSAGE_NOT_FOUND cases)
- [x] `npx jest agent-role.service` (9/9)
- [x] `npx oxlint --type-aware` on all changed files (0 warnings/errors)
- [x] `npx prettier --check` on all changed files
- [ ] CI

packages/twenty-server/src/engine/metadata-modules/ai/ai.exception.ts

@@ -0,0 +1,61 @@
+import { type MessageDescriptor } from '@lingui/core';
+import { msg } from '@lingui/core/macro';
+import { assertUnreachable } from 'twenty-shared/utils';
+
+import { CustomException } from 'src/utils/custom-exception';
+
+export enum AiExceptionCode {
+  AGENT_NOT_FOUND = 'AGENT_NOT_FOUND',
+  AGENT_ALREADY_EXISTS = 'AGENT_ALREADY_EXISTS',
+  AGENT_IS_STANDARD = 'AGENT_IS_STANDARD',
+  AGENT_EXECUTION_FAILED = 'AGENT_EXECUTION_FAILED',
+  INVALID_AGENT_INPUT = 'INVALID_AGENT_INPUT',
+  THREAD_NOT_FOUND = 'THREAD_NOT_FOUND',
+  MESSAGE_NOT_FOUND = 'MESSAGE_NOT_FOUND',
+  API_KEY_NOT_CONFIGURED = 'API_KEY_NOT_CONFIGURED',
+  USER_WORKSPACE_ID_NOT_FOUND = 'USER_WORKSPACE_ID_NOT_FOUND',
+  ROLE_NOT_FOUND = 'ROLE_NOT_FOUND',
+  ROLE_CANNOT_BE_ASSIGNED_TO_AGENTS = 'ROLE_CANNOT_BE_ASSIGNED_TO_AGENTS',
+}
+
+const getAiExceptionUserFriendlyMessage = (code: AiExceptionCode) => {
+  switch (code) {
+    case AiExceptionCode.AGENT_NOT_FOUND:
+      return msg`Agent not found.`;
+    case AiExceptionCode.AGENT_ALREADY_EXISTS:
+      return msg`An agent with this name already exists.`;
+    case AiExceptionCode.AGENT_IS_STANDARD:
+      return msg`Standard agents cannot be modified.`;
+    case AiExceptionCode.AGENT_EXECUTION_FAILED:
+      return msg`Agent execution failed.`;
+    case AiExceptionCode.INVALID_AGENT_INPUT:
+      return msg`Invalid agent input.`;
+    case AiExceptionCode.THREAD_NOT_FOUND:
+      return msg`Chat thread not found.`;
+    case AiExceptionCode.MESSAGE_NOT_FOUND:
+      return msg`Chat message not found.`;
+    case AiExceptionCode.API_KEY_NOT_CONFIGURED:
+      return msg`API key is not configured.`;
+    case AiExceptionCode.USER_WORKSPACE_ID_NOT_FOUND:
+      return msg`User workspace not found.`;
+    case AiExceptionCode.ROLE_NOT_FOUND:
+      return msg`Role not found.`;
+    case AiExceptionCode.ROLE_CANNOT_BE_ASSIGNED_TO_AGENTS:
+      return msg`This role cannot be assigned to agents.`;
+    default:
+      assertUnreachable(code);
+  }
+};
+
+export class AiException extends CustomException<AiExceptionCode> {
+  constructor(
+    message: string,
+    code: AiExceptionCode,
+    { userFriendlyMessage }: { userFriendlyMessage?: MessageDescriptor } = {},
+  ) {
+    super(message, code, {
+      userFriendlyMessage:
+        userFriendlyMessage ?? getAiExceptionUserFriendlyMessage(code),
+    });
+  }
+}