fix: return 409 Conflict instead of 502 when task agent is busy (#23424)

The "Task app is not ready to accept input" error occurs when the
agent responds successfully but its status is not "stable" (e.g.
"running"). This is a state conflict, not a gateway error. 502 was
semantically wrong because the gateway communication succeeded.

409 Conflict is correct because the request conflicts with the
agent's current state. This is consistent with how
authAndDoWithTaskAppClient already returns 409 for pending,
initializing, and paused agent states.

coderd/aitasks_test.go

@@ -789,6 +789,11 @@ func TestTasks(t *testing.T) {
 			})
 			require.Error(t, err, "wanted error due to bad status")
 
+			var sdkErr *codersdk.Error
+			require.ErrorAs(t, err, &sdkErr)
+			require.Equal(t, http.StatusConflict, sdkErr.StatusCode())
+			require.Contains(t, sdkErr.Message, "not ready to accept input")
+
 			statusResponse = agentapisdk.StatusStable
 
 			//nolint:tparallel // Not intended to run in parallel.