pageserver: mark tenant broken when cancelling attach (#6430)

## Problem

When a tenant is in Attaching state, and waiting for the
`concurrent_tenant_warmup` semaphore, it also listens for the tenant
cancellation token. When that token fires, Tenant::attach drops out.
Meanwhile, Tenant::set_stopping waits forever for the tenant to exit
Attaching state.

Fixes: https://github.com/neondatabase/neon/issues/6423

## Summary of changes

- In the absence of a valid state for the tenant, it is set to Broken in
this path. A more elegant solution will require more refactoring, beyond
this minimal fix.

pageserver/src/tenant.rs

@@ -716,6 +716,10 @@ impl Tenant {
                             // stayed in Activating for such a long time that shutdown found it in
                             // that state.
                             tracing::info!(state=%tenant_clone.current_state(), "Tenant shut down before activation");
+                            // Make the tenant broken so that set_stopping will not hang waiting for it to leave
+                            // the Attaching state.  This is an over-reaction (nothing really broke, the tenant is
+                            // just shutting down), but ensures progress.
+                            make_broken(&tenant_clone, anyhow::anyhow!("Shut down while Attaching"));
                             return Ok(());
                         },
                     )