feat: implement comprehensive testing framework and regression suite

- Established a centralized test specification in docs/tests.md that defines test categories, cases, and a coverage checklist to ensure consistent quality and maintainability across the project.

- Implemented RegressionTestExecutor for managing automated regression tests on a live Zabbix system.

- Updated GraphQL schema and resolvers with a generic runAllRegressionTests mutation.

- Enhanced MCP integration with new operation files and detailed documentation for AI-driven automation.

- Updated README.md and How-To guides (Cookbook, Maintenance, MCP) to reflect the new testing framework and MCP capabilities.

- Verified all changes with a full Jest suite (74 tests) and live end-to-end smoketests.

schema/mutations.graphql

@@ -138,6 +138,16 @@ type Mutation {
         """Technical name for the smoketest host group."""
         groupName: String!
     ): SmoketestResponse!
+
+    """
+    Runs all regression tests.
+    """
+    runAllRegressionTests(
+        """Technical name for the test host."""
+        hostName: String!, 
+        """Technical name for the test host group."""
+        groupName: String!
+    ): SmoketestResponse!
 }
 
 """