fix(presets): reject falsy malformed catalog roots on load

[marcelsafin]

Description

PresetCatalog._load_catalog_config() used yaml.safe_load(...) or {}, so falsy non-mapping documents such as [], 0, false, and an empty string were silently accepted as empty configuration. That hides corrupt catalog files and makes them behave differently from truthy malformed roots.

This preserves the existing empty/null no-op while rejecting every present non-mapping root consistently.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest (6,655 passed, 177 skipped)
• Added regressions for four falsy non-mapping YAML roots
• uvx ruff@0.15.0 check src tests clean

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Implemented autonomously by GitHub Copilot CLI (model: GPT-5.6 Sol) under human direction; failing regressions were added before the fix, then the full suite and lint were run locally. Commit includes Assisted-by and Co-authored-by trailers.

[Copilot]

Pull request overview

Corrects preset catalog parsing so falsy malformed YAML roots are rejected while empty/null documents remain no-ops.

Changes:

• Distinguishes None from other falsy YAML values.
• Adds regression coverage for lists, booleans, numbers, and strings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/specify_cli/presets/__init__.py	Validates all present catalog roots as mappings.
tests/test_presets.py	Tests rejection of falsy non-mapping roots.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.