refactor(mcp): address review findings for list/get report tools

- Register report model type in get_schema (Fix #1): add _get_report_schema_core
  factory + "report" entry in _SCHEMA_CORE_FACTORIES; ModelType now includes "report"
- Add OwnedByMeMixin/CreatedByMeMixin to ListReportsRequest (Fix #2)
- DRY up list_reports.py column constants (Fix #3): import REPORT_* constants and
  get_report_columns from schema_discovery; pass created_by_me/owned_by_me to run_tool
- Extend test coverage (Fix #6): humanized timestamp fields, invalid order_column
  guard, owned_by_me/created_by_me DAO filter injection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>