test: add dot/double-dot rejection tests for sanitize_path_id

Addresses CR feedback on a2aproject#1115: explicitly test that '.' and '..'
are rejected by sanitize_path_id to prevent path traversal risks.

Author: Linux2010

tests/utils/test_sanitizers.py

@@ -90,6 +90,17 @@ def test_path_traversal_rejected(self) -> None:
         with pytest.raises(InvalidRequestError, match='invalid characters'):
             sanitize_path_id('../../etc/passwd')
 
+    def test_dot_and_double_dot_rejected(self) -> None:
+        """Single dot and double dot are rejected as path traversal risks."""
+        with pytest.raises(
+            InvalidRequestError, match=r'cannot be "\." or "\.\."'
+        ):
+            sanitize_path_id('.')
+        with pytest.raises(
+            InvalidRequestError, match=r'cannot be "\." or "\.\."'
+        ):
+            sanitize_path_id('..')
+
     def test_custom_param_name_in_error(self) -> None:
         """Custom param_name appears in error messages."""
         with pytest.raises(InvalidRequestError, match='push_id'):