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/role/role.resolver.ts

@@ -28,9 +28,9 @@ import { UserAuthGuard } from 'src/engine/guards/user-auth.guard';
 import { WorkspaceAuthGuard } from 'src/engine/guards/workspace-auth.guard';
 import { AiAgentRoleService } from 'src/engine/metadata-modules/ai/ai-agent-role/ai-agent-role.service';
 import {
-  AgentException,
-  AgentExceptionCode,
-} from 'src/engine/metadata-modules/ai/ai-agent/agent.exception';
+  AiException,
+  AiExceptionCode,
+} from 'src/engine/metadata-modules/ai/ai.exception';
 import { AgentDTO } from 'src/engine/metadata-modules/ai/ai-agent/dtos/agent.dto';
 import { fromFlatAgentWithRoleIdToAgentDto } from 'src/engine/metadata-modules/flat-agent/utils/from-agent-entity-to-agent-dto.util';
 import { FieldPermissionDTO } from 'src/engine/metadata-modules/object-permission/dtos/field-permission.dto';
@@ -333,9 +333,9 @@ export class RoleResolver {
         flatApplicationMaps.byId[agentEntity.applicationId];
 
       if (!isDefined(flatApplication)) {
-        throw new AgentException(
+        throw new AiException(
           `Application not found for agent ${agentEntity.id}`,
-          AgentExceptionCode.AGENT_NOT_FOUND,
+          AiExceptionCode.AGENT_NOT_FOUND,
         );
       }