test: add E2E integration tests for converter

[mairas]

Description

Create end-to-end integration tests that validate full pipeline: CasaOS app → conversion → package → install → start.

MANDATORY: Follow docs/IMPLEMENTATION_CHECKLIST.md - TDD workflow required.

Implementation Phases

Explore → Plan → TEST FIRST → Implement Infrastructure → Verify → Commit

What to Build

• Write tests/converters/casaos/test_e2e.py FIRST
• Create E2E test infrastructure
• Set up Docker test environment
• Implement full pipeline tests

Acceptance Criteria

• E2E tests written
• Tests run in Docker
• Full pipeline tested (convert → package → install → start)
• Tests pass in CI
• Tests tagged appropriately

Dependencies

All implementation tasks (#81-90) complete