feat: expose MitoRegion::all_manifest_files for metadata rebuild (#8680)

* feat: expose MitoRegion::all_manifest_files for metadata rebuild

Adds a public read-only accessor that returns all live SST file metas
and the current manifest version from the region manifest. Used by the
downstream project admin path to enumerate the authoritative live file
set without going through the worker loop.

* fix(mito2): merge staging manifest files in all_manifest_files

The original implementation only read the normal manifest
(manifest_ctx.manifest()) and skipped staging_manifest(). While the
region is in staging mode (e.g. region copy/migration), the authoritative
live file set lives in the staging manifest, so callers would silently
miss those files.

Now matches the semantics of manifest_sst_entries() (~L771), which
explicitly merges manifest().files with staging_manifest().files via a
HashMap collect (dedup by FileId). The returned manifest version is the
staging version when a staging manifest is present, otherwise the normal
manifest version.

Also removed downstream-specific references from the rustdoc comments.

src/mito2/src/region.rs

@@ -849,6 +849,44 @@ impl MitoRegion {
             .collect::<Vec<_>>()
     }
 
+    /// Returns all live SST file metas and the current manifest version from
+    /// the region manifest, merging both the normal and staging manifests.
+    ///
+    /// While the region is in staging mode (e.g. during region copy/migration),
+    /// the authoritative live file set lives in the staging manifest, so this
+    /// method merges both — matching the semantics of [`manifest_sst_entries`].
+    ///
+    /// The returned manifest version is the staging version when a staging
+    /// manifest is present, otherwise the normal manifest version.
+    pub async fn all_manifest_files(&self) -> (Vec<FileMeta>, ManifestVersion) {
+        let manifest = self.manifest_ctx.manifest().await;
+        let staging = self
+            .manifest_ctx
+            .staging_manifest()
+            .await
+            .map(|m| (m.files.clone(), m.manifest_version));
+
+        let version = staging
+            .as_ref()
+            .map(|(_, v)| *v)
+            .unwrap_or(manifest.manifest_version);
+
+        let files = match staging {
+            Some((staging_files, _)) => {
+                let merged = manifest
+                    .files
+                    .clone()
+                    .into_iter()
+                    .chain(staging_files)
+                    .collect::<std::collections::HashMap<_, _>>();
+                merged.into_values().collect()
+            }
+            None => manifest.files.values().cloned().collect(),
+        };
+
+        (files, version)
+    }
+
     /// Exit staging mode successfully by merging all staged manifests and making them visible.
     /// Merges staged manifest actions into the live manifest and exits staging mode.
     ///