test: add 35 comprehensive tests covering strategies, e2e pipeline, edge cases, and ABI types

Expands test suite from 15 to 48 integration tests (50 total with unit tests):
- Point-to-point: one_for_one, one_for_all, rest_for_one strategy codegen
- End-to-end: full pipeline and minimal no-supervisors pipeline
- Edge cases: deep nesting, empty supervisors, all restart types, boundary values, invalid inputs
- Aspect: CamelCase naming, mix.exs references, diagram validity, ExUnit usage, SPDX headers
- ABI: ProcessTree depth/size/worker_count, enum repr values, RestartIntensity defaults

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath