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.

mcp/operations/runAllRegressionTests.graphql

@@ -0,0 +1,13 @@
+# Runs all regression tests.
+# Variables: hostName, groupName
+mutation RunAllRegressionTests($hostName: String!, $groupName: String!) {
+  runAllRegressionTests(hostName: $hostName, groupName: $groupName) {
+    success
+    message
+    steps {
+      name
+      success
+      message
+    }
+  }
+}