fix: set non-zero exit code when errors are found in analyzer

[assassinaj602]

Description

Fixes issue where dartdoc_test returns exit code 0 even when tests fail.

The Problem

When running dart run dartdoc_test, the command would always exit with code 0, even when errors were found. This made it impossible to use in CI/CD pipelines.

What I Changed

• Added import 'dart:io'; to lib/src/analyzer.dart
• Set exitCode = 1 when hasIssues is true

Testing

• ✅ All 18 tests pass
• ✅ Errors now properly set non-zero exit code
• ✅ Success still returns exit code 0

Closes #274