Add PHPStan static analysis setup

[Seraphim200001]

#21 Add PHPStan as a Composer dev dependency, configure phpstan.neon at level 6, add the Composer analyse script, and fix reported iterable type issues across the SDK and tests. Integration tests requiring valid tokens remain skipped.

📋 Description

Brief description of the changes made in this PR.

✨ Type of Change

• 🐛 Bug fix (fixes an issue)
• ✨ New feature (adds functionality)
• 🔧 Refactoring (code restructuring without functional changes)
• 📚 Documentation (updates to documentation)
• 🧪 Tests (adding or modifying tests)
• 🔒 Security (security-related fixes)
• ⚡ Performance (performance improvements)
• 🎨 Style (formatting changes, no logic changes)

🔍 Main Changes

Add PHPStan static analysis setup

• Add PHPStan as a Composer dev dependency
• Add phpstan.neon configuration with level 6
• Add Composer analyse script
• Fix PHPStan iterable type issues across the SDK and tests
• Keep integration tests skipped when valid tokens are required

🧪 Testing

• I have tested the changes locally
• I have added/updated unit tests
• All tests pass (./vendor/bin/phpunit)
• I have verified PHP 8.0+ compatibility

📝 Additional Notes

Any additional information, considerations, or important context for reviewers.

🔗 Related Issue

Closes #21 (if applicable)

📸 Screenshots (if applicable)

If changes affect the interface or visible behavior, include screenshots.

✅ Checklist

• My code follows the project conventions
• I have performed a self-review of my code
• I have commented complex code where necessary
• Documentation has been updated (if needed)
• I have not introduced breaking changes (or documented them)
• I have verified no sensitive information is in the code