test: add public API snapshot guard

[akmhatey-ai]

Summary

Adds a public API snapshot guard for chainweaver.__all__, exported class/function signatures, and exported Pydantic model fields. Prepared with AI assistance and verified locally.

Changes

• Add tests/public_api_snapshot.py and tests/test_public_api_snapshot.py.
• Add the initial deterministic fixture at tests/fixtures/public_api.json.
• Add tests/scripts/regen_public_api.py for intentional public API changes.
• Add the regeneration step to the review checklist.

Testing

• Linting passes (ruff check chainweaver/ tests/ examples/)
• Formatting check passes (ruff format --check chainweaver/ tests/ examples/)
• Type checking passes (python -m mypy chainweaver/ tests/)
• All existing tests pass (python -m pytest tests/ -v)
• New tests added for new functionality

Additional validation:

• python tests/scripts/regen_public_api.py ran in 500.43 ms locally and produced a stable fixture hash on repeat regeneration.
• Smoke check confirmed the snapshot changes when a public symbol is removed, a public class signature changes, or a Pydantic model field changes.
• git diff --cached --check
• git diff --cached | gitleaks detect --pipe --redact --verbose --no-color

Related Issues

Closes #140

Checklist

• Code follows project conventions (see AGENTS.md and docs/agent-context/)
• Public API changes are documented
• No secrets or credentials included

Limitations: this intentionally does not snapshot docstrings, matching the issue scope.