Add the `list` subcommand to the validation CLI command group. This fixes issue #8621 where users were unable to list registered validations through the CLI interface due to the command not being implemented and registered.

Changes:
- Added `list` command with --namespace/-n, --source/-s, and --pattern/-p (regex) filters for listing validation agents
- Extracted shared helper functions into new validation_helpers.py module
- Updated imports in validation.py to use extracted helpers instead of inline private functions
- Fixed lint and formatting issues

Testing:
- Added BDD regression test feature file (features/validation_list_command.feature) with 7 scenarios covering: empty state, rich table display, namespace/source/pattern filtering, JSON output, and YAML output modes
- Added corresponding step definitions in features/steps/validation_list_command_steps.py

ISSUES CLOSED: #8621

@@ -8,0 +15,4 @@
+  definitions, provider extraction from type names via prefix matching, three BDD
+  feature files (`cloud_resources.feature`, `cloud_handler_coverage.feature`,
+  `cloud_handler_coverage_r3.feature`) covering credential masking, inheritance
+  chains, sandbox strategies, and full ResourceHandler Protocol conformance testing

@@ -0,0 +70,4 @@
+        ),
+        _make_validation(
+            "global/security-scan",
+            "Security vulnerability scan",

@@ -0,0 +80,4 @@
+    mock_service.list_tools.return_value = validations
+    mock_service.get_tool.return_value = None
+
+    from cleveragents.cli.commands.validation_app import app as validation_app  # noqa: F401

@@ -0,0 +209,4 @@
+@when('I run the validation list command with format "json"')
+def step_run_list_json(context: Any) -> None:
+    """Run the list command with JSON output format."""
+    from cleveragents.cli.main import app as main_app

@@ -8,0 +17,4 @@
+  `cloud_handler_coverage_r3.feature`) covering credential masking, inheritance
+  chains, sandbox strategies, and full ResourceHandler Protocol conformance testing
+  for CRUD and lifecycle stub methods (read, write, delete, list_children, diff,
+  discover_children, create_sandbox, create_checkpoint, rollback_to, project_access).

@@ -0,0 +1,304 @@
+"""BDD step definitions for the 'agents validation list' command."""

@@ -0,0 +1,125 @@
+"""Helper functions for validation CLI commands.