feat: add comprehensive test suite for ExploreDNS

- Unit tests for all internal packages exceeding 80% coverage:
  - internal/config: 86.2% (ParseMaxDepth, ParseRetries, validation paths)
  - internal/dns: 82.9% (IterativeQueryWithExchange, mock DNS server, roots)
  - internal/fingerprint: 96.4% (unchanged, already excellent)
  - internal/output: 86.7% (formatters, stats, JSON/text output, hooks)
  - internal/traverse: 86.7% (SetHooks, ResolveNS, processReferral,
    ensureRDFalse, resolveGlueViaSystem, newAQuery, Referral.Resolve)

- Integration tests in internal/integration/:
  - End-to-end traversal with mock DNS exchange function
  - Referral chain traversal (root -> TLD -> authoritative)
  - CNAME resolution and loop detection
  - NXDOMAIN and SERVFAIL response handling
  - Max depth enforcement
  - Context cancellation
  - TraverserHooks event delivery

- Mock DNS server helper in internal/dns/roots_test.go using miekg/dns
  (enables deterministic testing without network dependency)

- CI updated with coverage reporting step
- Makefile: added 'cover' target for local HTML coverage reports

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

.gitea/workflows/ci.yml

@@ -23,5 +23,13 @@ jobs:
       - name: Test
         run: go test -v -race -coverprofile=coverage.out ./...
 
+      - name: Coverage report
+        run: go tool cover -func=coverage.out
+
+      - name: Check internal package coverage
+        run: |
+          COVERAGE=$(go tool cover -func=coverage.out | grep "^github.com/hits/ExploreDNS/internal" | awk '{print $3}' | sed 's/%//' | awk '{sum+=$1; count++} END {if(count>0) print sum/count; else print 0}')
+          echo "Average internal package coverage: ${COVERAGE}%"
+
       - name: Build
         run: go build -v ./...