Harden diagram IR edge rendering

[ZimoLiao]

What does this PR do?

• Hardens diagram --from-ir so IR edges using common source / target aliases render correctly alongside the canonical from / to keys.
• Converts malformed edge schemas into clean ValueError messages instead of raw tracebacks.
• Adds regression coverage and a changelog entry from the CLI smoke pass.

Type of change

• Bug fix
• New feature / skill
• Refactoring (no behavior change)
• Documentation
• CI / tooling

Checklist

• ruff check passes
• pytest passes
• New public API has docstrings (N/A; no new public API)
• CHANGELOG.md updated (for user-facing changes)

Verification

• PYTHONDONTWRITEBYTECODE=1 python -m pytest -q -p no:cacheprovider tests/test_diagram.py tests/test_diagram_integration.py tests/test_diagram_fault_injection.py -> 112 passed, 3 skipped
• PYTHONDONTWRITEBYTECODE=1 python -m pytest -q -p no:cacheprovider -> 1311 passed, 3 skipped
• PYTHONDONTWRITEBYTECODE=1 python -m ruff check scholaraio/services/diagram.py tests/test_diagram.py -> All checks passed
• PYTHONDONTWRITEBYTECODE=1 python -m ruff format --check scholaraio/services/diagram.py tests/test_diagram.py -> 2 files already formatted
• Manual CLI smoke reproduced diagram --from-ir ... --format mermaid with source / target edges successfully.