fix: add missing_key error kind for missing chat api_key_id (#25783)

Refs CODAGT-486

- `codersdk/chats.go`: New `ChatErrorKindMissingKey` constant and
`AllChatErrorKinds` entry
- `coderd/x/chatd/chaterror/message.go`: `terminalMessage` and
`retryMessage` cases
- `coderd/x/chatd/model_routing_aibridge.go`: Pre-classify error with
`WithClassification`
- `coderd/x/chatd/model_routing_internal_test.go`: Classification
assertion on production path (CRF-2)
- `chatStatusHelpers.ts`: Frontend title "Chat interrupted"
- `LiveStreamTail.stories.tsx`: Storybook story with `detail` assertion
- `docs/ai-coder/ai-gateway/clients/coder-agents.md`: Troubleshooting
entry
- Tests: classification round-trip, terminal message, metrics kind
enumeration

> Generated with [Coder Agents](https://coder.com/agents) on behalf of
@johnstcn

docs/ai-coder/ai-gateway/clients/coder-agents.md

@@ -164,6 +164,13 @@ key is a valid Coder token.
   one [model](../../agents/models.md#add-a-model) to the provider after
   saving the Base URL. Providers without an enabled model are hidden from
   developers.
+- **"Chat interrupted" error when resuming a conversation.**
+  This occurs when the API key that was used to start a chat turn is no
+  longer available. Common causes: upgrading from a version before
+  `api_key_id` tracking was introduced, or deleting an API key while a
+  chat is active. The error is self-healing: send your message again and
+  the new message will use your current API key. If the error persists
+  after resending, this indicates a bug. Please report it.
 
 ## Known limitations