fix(extensions): restore core-command discovery broken by #3014 module move

[dhruv-15-03]

What

Fix the off-by-one path resolution in specify_cli.extensions._load_core_command_names() so it actually discovers the bundled core command templates instead of silently returning the hardcoded fallback set. Fixes #3294.

Why

When this code moved from src/specify_cli/extensions.py to src/specify_cli/extensions/__init__.py (#3014, 826e193), the file went one directory deeper but the bespoke Path(__file__) parent counts were not updated. Both candidate dirs now resolve to non-existent paths:

• wheel -> specify_cli/extensions/core_pack/commands (real: specify_cli/core_pack/commands)
• source -> src/templates/commands (real: repo-root templates/commands)

So _load_core_command_names() always falls through to _FALLBACK_CORE_COMMAND_NAMES. It only looks healthy because the fallback currently equals the 10 real command stems. CORE_COMMAND_NAMES (built from this function) guards extensions against shadowing core commands (#1994); with discovery dead, that guard depends on the fallback being hand-maintained -- converge was manually added to it in #3001 (0c29d89).

Same off-by-one class @mnriem diagnosed for the presets loader (re #3086, #2826), but in a module the preset-scoped fix does not touch.

How

• Delegate path resolution to the canonical _assets resolvers (_locate_core_pack / _repo_root), exactly as presets/__init__.py does. These are anchored to the package root, so discovery survives future module moves.
• Add tests/test_extensions.py::TestCoreCommandDiscovery covering the source, wheel, and fallback branches, plus a sentinel test proving discovery returns the real bundled set rather than the fallback.

Verification

• New regression tests fail on the pre-fix code (the sentinel test fails with a clean assertion -- it returns the fallback sentinel instead of the bundled set) and pass after the fix.
• tests/test_extensions.py: 328 passed, 3 skipped.
• Broader relevant suites (imports, extensions, presets, integration registry/manifest): 887 passed. The only failures are 6 pre-existing Windows symlink-privilege errors in test_manifest.py that fail identically on the unmodified base (unrelated to this change).

Behavior change

None for end users today (the fallback already matches the real commands). This restores the intended dynamic discovery and removes the silent manual-maintenance dependency on the fallback list.

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Pull request overview

This PR fixes dynamic discovery of bundled core command templates used by the extensions system to prevent core-command shadowing. It corrects broken path resolution introduced when the extensions module moved into a package, by delegating lookup to the canonical _assets helpers and adding regression tests that ensure discovery doesn’t silently fall back to a hardcoded list.

Changes:

• Update _load_core_command_names() to resolve command template directories via _locate_core_pack() (wheel) and _repo_root() (source) instead of Path(__file__) math.
• Add regression tests covering: real discovery vs fallback, source-tree discovery, wheel core_pack preference, and fallback behavior/lockstep.

Reviewed changes

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File	Description
src/specify_cli/extensions/__init__.py	Fixes core-command discovery by using _assets path resolvers (wheel + source) instead of brittle relative parent traversal.
tests/test_extensions.py	Adds regression tests to ensure discovery reads real bundled templates and doesn’t silently rely on the fallback set.

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Review details

• Files reviewed: 2/2 changed files
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• Review effort level: Low