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

coderd/apidoc/swagger.json

@@ -14904,7 +14904,8 @@
 				"startup_timeout",
 				"auth",
 				"config",
-				"usage_limit"
+				"usage_limit",
+				"missing_key"
 			],
 			"x-enum-varnames": [
 				"ChatErrorKindGeneric",
@@ -14914,7 +14915,8 @@
 				"ChatErrorKindStartupTimeout",
 				"ChatErrorKindAuth",
 				"ChatErrorKindConfig",
-				"ChatErrorKindUsageLimit"
+				"ChatErrorKindUsageLimit",
+				"ChatErrorKindMissingKey"
 			]
 		},
 		"codersdk.ChatFileMetadata": {