refactor: move chat error kinds into codersdk (#24955)

Moves the chat error kind taxonomy from `coderd/x/chatd/chaterror` into
`codersdk.ChatErrorKind` and types `ChatError.Kind` /
`ChatStreamRetry.Kind` so generated TypeScript exposes an SDK-owned
union, including `usage_limit`. Backend chat classification now
references the SDK constants directly while preserving the existing JSON
string values.

Keeps chat usage-limit admission failures on their existing 409 response
shape. The frontend maps structured usage-limit responses to the
SDK-owned `usage_limit` kind, uses generated `TypesGen.ChatErrorKind`
directly, and removes the local string union and alias.

enterprise/coderd/x/chatd/chatd_test.go

@@ -587,7 +587,7 @@ func TestSubscribeRetryEventAcrossInstances(t *testing.T) {
 	require.NotNil(t, retryEvent)
 	require.Equal(t, 1, retryEvent.Attempt)
 	require.Greater(t, retryEvent.DelayMs, int64(0))
-	require.Equal(t, "rate_limit", retryEvent.Kind)
+	require.Equal(t, codersdk.ChatErrorKindRateLimit, retryEvent.Kind)
 	require.Equal(t, "openai", retryEvent.Provider)
 	require.Equal(t, 429, retryEvent.StatusCode)
 	require.Contains(t, retryEvent.Error, "rate limiting requests")