fix!: persist structured chat errors (#24919)

**Breaking change for changelog:**

> `codersdk.Chat.last_error` now returns a structured `ChatError` object
(`{message, kind, provider, retryable, status_code, detail}`) instead of
a plain string. The chats API is experimental
(`/api/experimental/chats`), so this ships without a deprecation cycle;
consumers reading `chat.last_error` as a string must update to read
`chat.last_error.message`. SDK/generated TypeScript terminal error
payloads now use the single `ChatError` type; the live stream error
payload type is renamed from `ChatStreamError` to `ChatError`.

Persisted chat errors now carry the same provider-specific detail (kind,
provider, retryable, HTTP status, optional detail) as the live stream,
so refreshing a failed chat rehydrates with the full structured error
instead of a one-line headline.

Existing rows are migrated in place: legacy text errors are wrapped into
`{message, kind: "generic"}` so already-errored chats still render, and
rows with `last_error IS NULL` stay NULL. Internally, persisted fallback
decoding now reuses the existing `chaterror.KindGeneric` constant, with
no JSON value change.

Closes CODAGT-239

coderd/pubsub/chatstreamnotify.go

@@ -40,7 +40,7 @@ type ChatStreamNotifyMessage struct {
 	// ErrorPayload carries a structured error event for cross-replica
 	// live delivery. Keep Error for backward compatibility with older
 	// replicas during rolling deploys.
-	ErrorPayload *codersdk.ChatStreamError `json:"error_payload,omitempty"`
+	ErrorPayload *codersdk.ChatError `json:"error_payload,omitempty"`
 
 	// Error is the legacy string-only error payload kept for mixed-
 	// version compatibility during rollout.