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

[Copilot]

Pull request overview

Migrates ExtensionCatalog to inherit from the shared CatalogStackBase, dropping its private duplicates of catalog URL validation and YAML config parsing. CatalogEntry becomes a thin subclass of the shared BaseCatalogEntry to preserve public import compatibility, and built-in/SPECKIT_CATALOG_URL/project/user catalog paths now go through self._entry() and self.CONFIG_FILENAME. Test suite gains focused coverage for malformed extension catalog configs (non-mapping roots, boolean priorities, blank/null names, invalid URLs with context) and applies a couple of unrelated lint cleanups.

Changes:

• ExtensionCatalog now subclasses CatalogStackBase, declaring CONFIG_FILENAME/ENTRY_CLASS/ERROR_TYPE/VALIDATION_ERROR_TYPE, and removes its in-class _validate_catalog_url and _load_catalog_config.
• CatalogEntry is redefined as class CatalogEntry(BaseCatalogEntry) to keep the existing public symbol while inheriting fields from the shared dataclass.
• New parametrized tests cover non-mapping YAML roots, boolean priorities, blank/null name normalization, and URL error context for the extension catalog config; plus minor lint fixes (unused variable, split combined import).

Show a summary per file

File	Description
src/specify_cli/extensions.py	Reparent ExtensionCatalog on CatalogStackBase, switch entry construction to self._entry, drop duplicated URL/config helpers.
tests/test_extensions.py	Add hardening tests for malformed extension-catalog configs; small lint-only cleanups.

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• Files reviewed: 2/2 changed files
• Comments generated: 0