FIX: Destroy pending PM after an unexpected send error (#40461)

Previously, when `Scriptable::Utils.send_pm` raised an unexpected error,
`Jobs::DiscourseAutomation::Tracker#send_pending_pm` only logged it and
left the `PendingPm` row intact, so the scheduled job re-processed the
same failing PM every minute indefinitely.

This change destroys the pending PM in the generic `rescue` block —
mirroring the existing `ActiveRecord::RecordNotSaved` handling — so a
permanently-failing PM is removed after one attempt instead of looping
forever.

Resolves https://patch.discourse.org/patch-triage/14

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/automation/app/jobs/scheduled/discourse_automation/tracker.rb

@@ -56,6 +56,7 @@ module Jobs
         ::DiscourseAutomation::Logger.error(
           "Error sending pending PM '#{pending_pm.title}': #{e.message}",
         )
+        pending_pm.destroy!
       end
 
       def run_pending_automation(pending_automation)

plugins/automation/spec/jobs/scheduled/discourse_automation/tracker_spec.rb

@@ -141,6 +141,34 @@ describe Jobs::DiscourseAutomation::Tracker do
       end
     end
 
+    context "when send_pm raises an unexpected error" do
+      before { pending_pm.update!(execute_at: 2.hours.ago) }
+
+      it "destroys the pending pm so it isn't retried forever" do
+        DiscourseAutomation::Scriptable::Utils.stubs(:send_pm).raises(
+          StandardError.new("unexpected error"),
+        )
+
+        expect { Jobs::DiscourseAutomation::Tracker.new.execute }.to change {
+          automation.pending_pms.count
+        }.by(-1)
+      end
+    end
+
+    context "when send_pm raises RecordNotSaved" do
+      before { pending_pm.update!(execute_at: 2.hours.ago) }
+
+      it "destroys the pending pm" do
+        DiscourseAutomation::Scriptable::Utils.stubs(:send_pm).raises(
+          ActiveRecord::RecordNotSaved.new("not saved"),
+        )
+
+        expect { Jobs::DiscourseAutomation::Tracker.new.execute }.to change {
+          automation.pending_pms.count
+        }.by(-1)
+      end
+    end
+
     it "doesn't send multiple messages if the job is invoked multiple times concurrently" do
       pending_pm.update!(execute_at: 1.hour.from_now)
       expect do