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/database/models.go

@@ -4367,7 +4367,7 @@ type Chat struct {
 	RootChatID          uuid.NullUUID         `db:"root_chat_id" json:"root_chat_id"`
 	LastModelConfigID   uuid.UUID             `db:"last_model_config_id" json:"last_model_config_id"`
 	Archived            bool                  `db:"archived" json:"archived"`
-	LastError           sql.NullString        `db:"last_error" json:"last_error"`
+	LastError           pqtype.NullRawMessage `db:"last_error" json:"last_error"`
 	Mode                NullChatMode          `db:"mode" json:"mode"`
 	MCPServerIDs        []uuid.UUID           `db:"mcp_server_ids" json:"mcp_server_ids"`
 	Labels              StringMap             `db:"labels" json:"labels"`