Implement M4: comprehensive test coverage with 120 tests

Service layer (63 tests): certificate, agent, audit, job, notification,
policy, and renewal services with mock repositories covering threshold
alerting, deduplication, status transitions, and job processing.

Handler layer (46 tests): certificate and agent HTTP handlers using
httptest with mock service interfaces, covering success/error paths,
pagination, JSON marshaling, and path parameter extraction.

Integration (11 subtests): end-to-end certificate lifecycle test
exercising real services and Local CA issuer through HTTP API —
create cert, trigger renewal, process jobs, register agent, heartbeat,
verify audit trail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/handler/test_utils.go

@@ -0,0 +1,11 @@
+package handler
+
+import "errors"
+
+var (
+	// Mock errors for testing
+	ErrMockServiceFailed = errors.New("mock service error")
+	ErrMockNotFound      = errors.New("mock not found error")
+	ErrMockUnauthorized  = errors.New("mock unauthorized error")
+	ErrMockConflict      = errors.New("mock conflict error")
+)