control_plane: avoid feedback loop with /location_config if compute hook fails. (#6668)

## Problem

The existing behavior isn't exactly incorrect, but is operationally
risky: if the control plane compute hook breaks, then all the control
plane operations trying to call /location_config will end up retrying
forever, which could put more load on the system.

## Summary of changes

- Treat 404s as fatal errors to do fewer retries: a 404 either indicates
we have the wrong URL, or some control plane bug is failing to recognize
our tenant ID as existing.
- Do not return an error on reconcilation errors in a non-creating
/location_config response: this allows the control plane to finish its
Operation (and we will eventually retry the compute notification later)

control_plane/attachment_service/src/compute_hook.rs

@@ -240,7 +240,7 @@ impl ComputeHook {
         let client = reqwest::Client::new();
         backoff::retry(
             || self.do_notify_iteration(&client, url, &reconfigure_request, cancel),
-            |e| matches!(e, NotifyError::Fatal(_)),
+            |e| matches!(e, NotifyError::Fatal(_) | NotifyError::Unexpected(_)),
             3,
             10,
             "Send compute notification",

control_plane/attachment_service/src/service.rs

@@ -989,7 +989,15 @@ impl Service {
                 .collect();
         } else {
             // This was an update, wait for reconciliation
-            self.await_waiters(waiters).await?;
+            if let Err(e) = self.await_waiters(waiters).await {
+                // Do not treat a reconcile error as fatal: we have already applied any requested
+                // Intent changes, and the reconcile can fail for external reasons like unavailable
+                // compute notification API.  In these cases, it is important that we do not
+                // cause the cloud control plane to retry forever on this API.
+                tracing::warn!(
+                    "Failed to reconcile after /location_config: {e}, returning success anyway"
+                );
+            }
         }
 
         Ok(result)