fix: remove unnecessary pass statements from exception classes

A class body with just a docstring is valid Python -- the pass
statements were noise.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/exceptions.py

@@ -4,25 +4,21 @@
 class DqliteError(Exception):
     """Base exception for dqlite client errors."""
 
-    pass
 
 
 class DqliteConnectionError(DqliteError):
     """Error establishing or maintaining connection."""
 
-    pass
 
 
 class ProtocolError(DqliteError):
     """Protocol-level error."""
 
-    pass
 
 
 class ClusterError(DqliteError):
     """Cluster-related error (leader not found, etc)."""
 
-    pass
 
 
 class OperationalError(DqliteError):