refactor: migrate ExtensionCatalog to shared catalog stack base

[WOLIKIMCHENG]

Summary

• Migrate ExtensionCatalog to inherit CatalogStackBase
• Remove duplicate extension catalog URL validation and YAML stack parsing
• Preserve extension catalog source ordering and public CatalogEntry import compatibility
• Add focused tests for malformed extension catalog config hardening

Behavior Notes

Valid extension catalog configs should behave the same. Malformed configs now align with shared base behavior:

• non-mapping YAML roots raise ValidationError
• boolean priorities are rejected
• blank/null names normalize to catalog-N
• invalid catalog URLs include config path and entry index context

Tests

• uv run --with ruff ruff check src/specify_cli/extensions.py tests/test_extensions.py
  • Passed: all checks passed
• Focused pytest selection for extension catalog stack, ExtensionCatalog, CLI path output, update hardening, and touched remediation test
  • Passed: 49 passed

Closes #2437