fix: prevent extension self-install from deleting source dir (#2990)

[jinaparkdev]

specify extension add <path> --dev --force permanently deleted the extension directory without registering it when the source path resolved to the extension's own install location
(.specify/extensions/<id>).

With --force, install_from_directory() removed the existing installation (the source) and then shutil.copytree() tried to copy from the now-deleted directory, destroying it
and crashing.

Add a guard that fails fast with a clear ValidationError when the resolved source path equals the install destination, before any destructive operation runs. Includes a regression
test asserting the directory and its contents survive.

Description

When the source path passed to extension add --dev --force resolves to the extension's own install destination, the old flow removed the source and then copied from a directory
that no longer existed — destroying the extension with no registration. This PR adds an early source == destination check in ExtensionManager.install_from_directory()
(src/specify_cli/extensions.py) that raises a clear ValidationError before any removal/copy happens, so the directory is never touched on the error path.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Manual end-to-end on macOS/zsh:

Scenario	Result
extension add .specify/extensions/<id> --dev --force (self-install) — with fix	✅ Clear error, directory + extension.yml + commands left intact
Same command without fix (reverted)	Reproduced original bug: FileNotFoundError, directory destroyed
extension add /other/path --dev --force (different location)	✅ Still installs — no regression

Added regression test test_install_from_install_dir_is_rejected_without_data_loss; full suite tests/test_extensions.py → 283 passed.

AI Disclosure

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

Developed with Claude Code (AI). The AI located the root cause, wrote the guard and the regression test. I reviewed every change, reproduced the original data-loss bug locally, and
verified the fix end-to-end myself before submitting.