feat: add PR body generator from agent bundle

[ProfRandom92]

Adds a deterministic Markdown PR body generator from agent artifact bundles.

Validation:

• python -m compileall -q scripts/pr_body_from_agent_bundle.py
• pytest tests/test_pr_body_from_agent_bundle.py -q

[gemini-code-assist]

Code Review

This pull request introduces a script and associated tests to generate a deterministic Markdown pull request body from an agent artifact bundle. The feedback focuses on improving the robustness of JSON parsing by enforcing strict type checking for list fields—specifically validation_evidence, problems, and changed_files—recommending that a RuntimeError be raised for non-list types instead of using silent fallbacks.

[gemini-code-assist]

The current implementation performs a silent fallback if validation_evidence is not a list. According to the general rules, non-list types for expected list fields should raise a RuntimeError to maintain strictness, while null or missing values should be treated as empty lists.

Suggested change

	if not isinstance(validation_evidence, list) or not validation_evidence:
	return ["- No validation evidence provided in bundle."]
	if validation_evidence is not None and not isinstance(validation_evidence, list):
	raise RuntimeError("validation_evidence must be a list")
	if not validation_evidence:
	return ["- No validation evidence provided in bundle."]

References

1. When processing JSON data, treat null or missing values for expected list fields as empty lists, but raise a RuntimeError for other non-list types to maintain strictness.

[gemini-code-assist]

The problems field should follow the strictness rule for expected list fields. If the field is present but is not a list, it should raise a RuntimeError instead of silently falling back to the default message. Additionally, use type guards for items within the list to prevent crashes on malformed data.

Suggested change

	problems = safe_pr_gate.get("problems")
	if isinstance(problems, list) and problems:
	lines.append("- problems:")
	lines.extend(f" - `{problem}`" for problem in problems if isinstance(problem, str))
	else:
	lines.append("- problems: `none`")
	problems = safe_pr_gate.get("problems")
	if problems is not None and not isinstance(problems, list):
	raise RuntimeError("safe_pr_gate.problems must be a list")
	if problems:
	lines.append("- problems:")
	lines.extend(f" - {problem}" for problem in problems if isinstance(problem, str))
	else:
	lines.append("- problems: none")

References

1. When processing JSON data, treat null or missing values for expected list fields as empty lists, but raise a RuntimeError for other non-list types to maintain strictness.
2. Use type guards like isinstance(item, dict) before accessing fields within list items to prevent crashes on malformed data.

[gemini-code-assist]

The changed_files field should also be handled strictly. If it is present but not a list, a RuntimeError should be raised per the repository's general rules.

Suggested change

	changed_files = bundle.get("changed_files")
	changed_file_lines = _bullet_list(changed_files if isinstance(changed_files, list) else [], "No changed files provided in bundle.")
	changed_files = bundle.get("changed_files")
	if changed_files is not None and not isinstance(changed_files, list):
	raise RuntimeError("changed_files must be a list")
	changed_file_lines = _bullet_list(changed_files or [], "No changed files provided in bundle.")

References

1. When processing JSON data, treat null or missing values for expected list fields as empty lists, but raise a RuntimeError for other non-list types to maintain strictness.