Coerce stringified JSON arrays in tool parameters

MCP clients (Claude Code deferred tools, LiveKit agents, etc.)
sometimes serialize list parameters as JSON-encoded strings instead of
native arrays. Add lenient coercion so these calls succeed.

- Add generic coerce_stringified_list() utility
- Fix fetch_artifacts: accept Union[str, List[str]] + coerce
- Fix _normalize_optional_list: parse stringified JSON for paths/extensions
- Add 14 unit tests + 5 e2e tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rodion-m