Fix Transaction benchmark isolation with proper cleanup

The Transaction benchmark implementation was missing proper cleanup
in fork() and unfork() methods, unlike the Client implementation
which properly cleaned state between iterations.

This caused state pollution between benchmarks:
- Delete/Neo4j_Transaction couldn't find links created by previous
  benchmark iterations
- Panic: 'NotExists(4000)' at delete.rs:23

Solution: Add drop_table() calls in Transaction's fork() and unfork()
methods to ensure each benchmark iteration starts with a clean state,
matching the Client implementation's behavior.

Fixes part of #5

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: konard

rust/src/benched.rs

@@ -100,10 +100,13 @@ impl<'a, T: LinkType> Benched for Exclusive<Transaction<'a, T>> {
     }
 
     fn fork(&mut self) -> Fork<Self> {
+        // Clean up any existing data before benchmark to ensure isolation
+        let _ = self.drop_table();
         Fork(self)
     }
 
     unsafe fn unfork(&mut self) {
-        // Transaction cleanup handled by client
+        // Clean up after benchmark iteration
+        let _ = self.drop_table();
     }
 }