[opampsupervisor] avoid nil pointer access when reporting remote config status (open-telemetry#36849)

#### Description

When working on an unrelated PR I noticed that one of the opamp
supervisor e2e tests was randomly failing, due to a nil pointer access -
see
https://github.com/open-telemetry/opentelemetry-collector-contrib/actions/runs/12350739300/job/34464544343?pr=36844#step:8:647
This PR avoids this by using the `GetConfigHash()` method of the
`remoteConfig` instead of directly accessing the `ConfigHash` property.

#### Link to tracking issue

Fixes open-telemetry#36850

---------

Signed-off-by: Florian Bacher <[email protected]>

Author: bacherfl

.chloggen/opampsupervisor-nil-pointer.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: opampsupervisor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: avoid nil pointer access when reporting the last received remote config
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [ 36850 ]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

cmd/opampsupervisor/supervisor/supervisor.go

@@ -1243,7 +1243,7 @@ func (s *Supervisor) saveLastReceivedOwnTelemetrySettings(set *protobufs.Telemet
 
 func (s *Supervisor) reportConfigStatus(status protobufs.RemoteConfigStatuses, errorMessage string) {
 	err := s.opampClient.SetRemoteConfigStatus(&protobufs.RemoteConfigStatus{
-		LastRemoteConfigHash: s.remoteConfig.ConfigHash,
+		LastRemoteConfigHash: s.remoteConfig.GetConfigHash(),
 		Status:               status,
 		ErrorMessage:         errorMessage,
 	})