fix(extensions): reject non-mapping extension.yml config section

[Noor-ul-ain001]

Summary

• ConfigManager._get_extension_defaults() read an extension's extension.yml config.defaults via manifest_data.get("config", {}).get("defaults", {}) with no shape check on the intermediate config value. Shipped extensions (e.g. extensions/git/extension.yml) use a top-level config: { defaults: {...} } section — distinct from the already-validated provides.config list. A manifest with config: [] or config: "oops" made the chained .get() raise a bare AttributeError instead of degrading gracefully like every other malformed config source in this class.
• That crash was silently swallowed by should_execute_hook's blanket except Exception, so a hook's config.x is set condition permanently evaluated to False for the extension with no diagnostic — a hook that should run (or should raise a clear error) instead silently never fires.
• Mirrors the existing TestConfigManagerNonMappingYaml coverage for a non-mapping root of <id>-config.yml (already fixed), one level deeper in the manifest's own config section, which was previously unchecked. ExtensionManifest._validate() does not validate this top-level config field at all.

Test plan

• Added TestConfigManagerNonMappingManifestConfigSection to tests/test_extensions.py (5 tests: list config:, scalar config:, non-mapping config.defaults, valid-shape regression guard, and a HookExecutor._evaluate_condition non-crash check)
• Verified all 4 crash-reproducing tests fail without the fix (AttributeError: 'list'/'str' object has no attribute 'get') and pass with it — stashed only the source change (src/specify_cli/extensions/__init__.py), confirmed the failure, then restored it and confirmed the pass
• Ran tests/test_extensions.py full suite — 505 passed, 20 pre-existing failures (Windows symlink-elevation and env-specific; reproduced identically on the unmodified branch), 8 skipped — no regressions

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01FW9fAYsCBCAgdKWovtSyqt