Simplify testing architecture: remove reusable workflow, use simple template pattern

BREAKING CHANGE: Removed reusable-package-test.yml workflow in favor of simpler template approach

Why this change:
- Reusable workflow added unnecessary complexity (JSON parsing, workflow_call inputs)
- Template pattern is clearer: copy/paste/modify a working example
- Both approaches require similar effort (~15-20 min to add a package)
- When tests need customization, template is already there to modify
- Eliminates false dichotomy between 'simple' and 'complex' approaches

Changes:
1. Deleted: .github/workflows/reusable-package-test.yml (230 lines)
2. Converted: test-envoy.yml from reusable pattern to direct implementation (47→295 lines)
   - Now matches nginx pattern
   - Direct bash commands (no JSON arrays)
   - Easy to understand and modify
3. Created: template-package-test.yml - well-commented template for new packages
4. Updated: test-all-packages.yml - simplified to just call workflows directly

Benefits:
- Simpler mental model (one pattern, not two)
- Easier to debug (everything in one file)
- More flexible (customize without hitting abstractions limitations)
- Better DX (no JSON string escaping, no workflow_call complexity)

All tests remain the same - nginx (5 tests) and Envoy (4 tests) unchanged.

Author: pareenaverma