Fail closed when wizard source_paths is not a string array

[jmjava]

Problem

PR #129 required wizard bodies to be JSON objects. source_paths still used list(data.get("source_paths") or []). A string is iterable, so "README.md" became ['R','E','A',…] and looked like missing files. A list guidance / text later called .strip() and raised AttributeError. Focus paths items were str()-coerced.

Change

• require_json_str_list / require_json_string
• source_paths (when present) must be a JSON array of strings; missing / null still [] (hint-path fallback)
• guidance, segment_name, revision_notes, current_narration, mode, topic_label, segment_id, PUT text must be JSON strings when present
• Focus paths items must be strings

Tests

Local: ruff check src/ tests/ green; pytest tests/ 755 passed, 1 skipped; docgen benchmark meets src/docgen/benchmark_data/baseline.json (no --update-baseline).

• String source_paths → 400
• Non-string list items → 400
• List guidance / text → 400
• Integer focus path → 400

Out of scope

• Invalid JSON still becomes {} via get_json(silent=True)
• Wizard except Exception around narration_topic_label