feat(discover): match unscoped tags against dir-prefixed model short names

[coketaste]

Summary

• Per-directory models.json files prefix each model name with their directory
  (e.g. pyt_foo becomes dir/pyt_foo). This silently broke --tags pyt_foo for
  users who referenced models by their original flat name, since the exact-name
  match would no longer find them and no error was raised to explain why.
• select_models() now also falls back to matching the model's short name (the
  part after the last /) for unscoped tags, for both models.json-defined
  models and custom models from get_models_json.py. This is purely additive —
  scoped tags (scope/tag), tag-field matching, and flat (non-prefixed) model
  names are all unaffected.
• Added TestShortNameBackwardCompat covering: short-name resolution of a
  dir-prefixed model, flat names still working, short-name resolution for custom
  models, and scoped tags being unaffected by the new short-name fallback.
• Updated test_unscoped_tag_does_not_cross_scope_boundary (now
  test_unscoped_tag_matches_scoped_model_by_short_name), which previously
  asserted the old (pre-fix) behavior that unscoped tags could not cross scope
  boundaries — that assumption is exactly what this change intentionally
  relaxes for backward compatibility.
• Added test_ambiguous_short_name_matches_both_flat_and_prefixed_model to pin
  down current behavior for the edge case where both a flat model (foo) and a
  dir-prefixed model (dir/foo) share the same short name — today both are
  selected, since there's no precedence between an exact match and a
  short-name match.

Test plan

• pytest tests/unit/test_discover_models.py -v — all 18 tests pass

[Copilot]

Pull request overview

This PR restores backward compatibility for DiscoverModels.select_models() when per-directory models.json introduces directory-prefixed model names (e.g., dir/pyt_foo), by allowing unscoped --tags <name> inputs to also match a model’s short name (the segment after the last /). It also adds/updates unit tests to lock in the intended behavior, including scoped-tag behavior remaining unchanged.

Changes:

• Extend unscoped name-based model selection to also match model["name"].split("/")[-1] (and the equivalent for CustomModel.name).
• Update an existing unit test to reflect the new backward-compat behavior for unscoped tags matching scoped models by short name.
• Add a new test suite covering dir-prefixed models, flat names, custom models, scoped tags, and ambiguous short-name scenarios.

Reviewed changes

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

File	Description
src/madengine/utils/discover_models.py	Adds short-name fallback matching for unscoped --tags against both models.json models and CustomModels.
tests/unit/test_discover_models.py	Updates/expands unit tests to validate short-name backward compatibility and preserve scoped-tag semantics.

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