Assert the tenant to be active in unoffload_timeline (#9539)

Currently, all callers of `unoffload_timeline` ensure that the tenant
the unoffload operation is called on is active. We rely on it being
active as we activate the timeline below and don't want to race with the
activation code of the tenant (in the worst case, activating a timeline
twice).

Therefore, add this assertion.

Part of #8088

pageserver/src/tenant.rs

@@ -1830,6 +1830,18 @@ impl Tenant {
         ctx: RequestContext,
     ) -> Result<Arc<Timeline>, TimelineArchivalError> {
         info!("unoffloading timeline");
+
+        // We activate the timeline below manually, so this must be called on an active timeline.
+        // We expect callers of this function to ensure this.
+        match self.current_state() {
+            TenantState::Activating { .. }
+            | TenantState::Attaching
+            | TenantState::Broken { .. } => {
+                panic!("Timeline expected to be active")
+            }
+            TenantState::Stopping { .. } => return Err(TimelineArchivalError::Cancelled),
+            TenantState::Active => {}
+        }
         let cancel = self.cancel.clone();
 
         // Protect against concurrent attempts to use this TimelineId