chore(flowcontrol): address review feedback

- fallbackRequest: document that on the fallback path item.OriginalRequest().FlowKey()
  reports the fallback priority rather than the originally requested one, despite the
  method name. Note surfacing the original priority in metrics as a follow-up.
- Drop the redundant NotErrorIs assertion in the band-GC regression test;
  require.NoError already fails the test on any error.

Signed-off-by: Luke Van Drie <lukevandrie@google.com>

Author: LukeAVanDrie

pkg/epp/flowcontrol/controller/controller.go

@@ -282,6 +282,12 @@ func (fc *FlowController) EnqueueAndWait(
 
 // fallbackRequest wraps a FlowControlRequest to override its flow key, so a request that falls back to a different
 // priority is enqueued under the band that was actually leased rather than its original (unprovisioned) band.
+//
+// Trade-off: downstream consumers see this wrapper, so item.OriginalRequest().FlowKey() reports the fallback
+// priority rather than the requested one — despite the method name. This is intentional, since the item must be
+// leased, distributed, and enqueued consistently at the fallback priority. The originally requested priority
+// therefore survives only in the withConnectionWithFallback log; surfacing it in metrics is left as a follow-up
+// (a dedicated fallback counter labeled with the original priority).
 type fallbackRequest struct {
 	flowcontrol.FlowControlRequest
 	key flowcontrol.FlowKey

pkg/epp/flowcontrol/registry/registry_test.go

@@ -957,11 +957,10 @@ func TestFlowRegistry_PriorityBandGarbageCollection(t *testing.T) {
 		require.True(t, exists,
 			"Control-plane-desired band %d must survive GC after inactivity", desiredPrio)
 
-		// A follow-up request to the still-desired band must not be rejected/demoted.
+		// A follow-up request to the still-desired band must not be rejected with ErrPriorityBandNotFound.
 		err := h.fr.WithConnection(key, func(conn contracts.ActiveFlowConnection) error { return nil })
 		require.NoError(t, err,
 			"Request to a still-desired band must succeed after inactivity")
-		assert.NotErrorIs(t, err, contracts.ErrPriorityBandNotFound)
 	})
 
 	t.Run("ShouldCollectMultipleBands_InOneCycle", func(t *testing.T) {