pageserver: persist stripe size in tenant manifest for tenant_import (#11181)

## Problem

`tenant_import`, used to import an existing tenant from remote storage
into a storage controller for support and debugging, assumed
`DEFAULT_STRIPE_SIZE` since this can't be recovered from remote storage.
In #11168, we are changing the stripe size, which will break
`tenant_import`.

Resolves #11175.

## Summary of changes

* Add `stripe_size` to the tenant manifest.
* Add `TenantScanRemoteStorageShard::stripe_size` and return from
`tenant_scan_remote` if present.
* Recover the stripe size during`tenant_import`, or fall back to 32768
(the original default stripe size).
* Add tenant manifest compatibility snapshot:
`2025-04-08-pgv17-tenant-manifest-v1.tar.zst`

There are no cross-version concerns here, since unknown fields are
ignored during deserialization where relevant.

storage_controller/src/service.rs

@@ -6014,9 +6014,21 @@ impl Service {
             .max()
             .expect("We already validated >0 shards");
 
-        // FIXME: we have no way to recover the shard stripe size from contents of remote storage: this will
-        // only work if they were using the default stripe size.
-        let stripe_size = ShardParameters::DEFAULT_STRIPE_SIZE;
+        // Find the tenant's stripe size. This wasn't always persisted in the tenant manifest, so
+        // fall back to the original default stripe size of 32768 (256 MB) if it's not specified.
+        const ORIGINAL_STRIPE_SIZE: ShardStripeSize = ShardStripeSize(32768);
+        let stripe_size = scan_result
+            .shards
+            .iter()
+            .find(|s| s.tenant_shard_id.shard_count == shard_count && s.generation == generation)
+            .expect("we validated >0 shards above")
+            .stripe_size
+            .unwrap_or_else(|| {
+                if shard_count.count() > 1 {
+                    warn!("unknown stripe size, assuming {ORIGINAL_STRIPE_SIZE}");
+                }
+                ORIGINAL_STRIPE_SIZE
+            });
 
         let (response, waiters) = self
             .do_tenant_create(TenantCreateRequest {