refactor: extract shared controller helpers to common package

[digiserg]

Summary

Several constants and helper methods are duplicated across controller packages.

Duplicated constants

condTypeReady = "Ready" declared independently in:

• internal/controller/alerts/axonopsmetricalert_controller.go:38
• internal/controller/backups/axonopsbackup_controller.go:43
• internal/controller/kafka/axonopskafkatopic_controller.go:39

Duplicated helpers

setFailedCondition method has identical bodies in every controller across alerts, backups, and kafka packages.

Duplicated test helpers

createTestConnectionAndSecret is copied in alerts/test_helpers_test.go, backups/test_helpers_test.go, kafka/test_helpers_test.go.

Fix

1. Move condTypeReady to internal/controller/common/connection.go
2. Add a shared SetFailedCondition helper to common (using generics or client.Object interface)
3. Extract test helper to internal/testutil/connection.go (test-only package)

Acceptance Criteria

1. condTypeReady exists only in common package
2. No duplicated setFailedCondition bodies across packages
3. Test helper exists in one location, imported by all test suites
4. All tests pass

Priority

P2