config: collect_config_issues raises on parseable-but-malformed YAML — 500 from /api/config/validate (contract says "never raises") #119

Problem

collect_config_issues documents itself as "never raises" (src/profiles/core/config/diagnostics.py:7), but line 45 calls collect_issues(raw) outside any try. Three verified crashes on content ruamel parses fine:

Blast radius

• POST /api/config/validate → 500 (routes/config.py:187-203).
• POST /api/config/save: validate_issues runs before the force check (config.py:467-469), so even force-save cannot write such drafts — the user gets a 500 instead of a 400 issue list.

Suggested fix

1. In read_yaml_text/read_yaml, type-narrow to Mapping — raise ValueError("configuration root must be a mapping") otherwise (or return {} and let diagnostics report it).
2. In validator.collect_issues, guard isinstance(extends, str) and isinstance(raw, Mapping).
3. Belt-and-braces: wrap the collect_issues call in collect_config_issues and convert any exception into an {"severity": "error", ...} issue.