refactor(schema): change tables() method to return iterator over values

Change SchemaDef::tables() return type from &HashMap<String, TableDef>
to impl Iterator<Item = &TableDef> to provide a cleaner API that focuses
on the table definitions rather than exposing the internal storage structure.

Signed-off-by: Svetlin Ralchev <iamralch@users.noreply.github.com>

Author: iamralch

src/schema.rs

@@ -87,8 +87,8 @@ impl SchemaDef {
     }
 
     /// Get all table schemas
-    pub fn tables(&self) -> &HashMap<String, TableDef> {
-        &self.tables
+    pub fn tables(&self) -> impl Iterator<Item = &TableDef> {
+        self.tables.values()
     }
 
     /// Parse CREATE TABLE statement