fix(presets): reject malformed catalog roots during mutation

[marcelsafin]

Description

Preset catalog add/remove commands assume the YAML document root is a mapping. A sequence or scalar root therefore raises a raw type error instead of reporting corrupt configuration, and mutation must not rewrite that file.

This validates the root shape before reading or changing catalog entries. Empty and null documents retain their existing empty-config behavior.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest (6,653 passed, 177 skipped)
• Added parameterized add/remove regressions for 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

Validates preset catalog YAML roots before mutation, preventing raw type errors and preserving malformed files.

Changes:

• Rejects non-mapping roots for catalog add/remove.
• Adds regression coverage for both mutation commands.

Reviewed changes

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

File	Description
src/specify_cli/presets/_commands.py	Validates catalog root shape before mutation.
tests/test_presets.py	Verifies rejection without rewriting configuration.

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