[Fix] Resolve regex warnings

[colinthebomb1]

Overview:

This PR fixes a warning in regex expressions by using raw strings for regex expressions. The PR also fixes a warning for unused return statements in ast test cases.

Example warnings:

core/rule_parser.py:238
/home/runner/work/QueryBooster/QueryBooster/core/rule_parser.py:238: DeprecationWarning: invalid escape sequence '\(' 'markerStart': ['\(', '\{', '\['],

tests/test_ast.py::test_complete_query
  /opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/_pytest/python.py:170: PytestReturnNotNoneWarning: Test functions should return None, but tests/test_ast.py::test_complete_query returned <class 'core.ast.node.QueryNode'>.

Code Changes:

• Used raw strings for regex in rule_parser.py and rule_generator.py
• Removed unused returns in test_ast.py