Core tests open their store through local::memory_conn

Seven test modules each built a migrated in-memory store by hand: open, migrate,
and (in sharing) wrap it in a Db. local::memory_conn() is that, and
open_in_memory uses it too. Each module's db() is now one line, and the schema,
Connection and Mutex imports it needed are gone.

DRY pass #2, batch 2, F9 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/local/portable.rs

@@ -593,11 +593,9 @@ fn dirname(path: &str) -> &str {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::local::schema;
 
     fn store() -> (Connection, BlobStore, std::path::PathBuf) {
-        let conn = Connection::open_in_memory().unwrap();
-        schema::migrate(&conn).unwrap();
+        let conn = crate::local::memory_conn().unwrap();
         let dir = std::env::temp_dir().join(format!("inkwell-portable-{}", uuid::Uuid::new_v4()));
         let blobs = BlobStore::new(dir.clone()).unwrap();
         (conn, blobs, dir)