Fix: allow --select-model to plan a model deletion

[viniciusnunest]

Description

Fixes #5741

Right now if you delete a model file locally and try to plan its removal with
--select-model=<deleted-model>, you get:

PlanError: Selector did not return any models.

The problem is that expand_model_selections only looks at local models. Since
the file is gone, it finds nothing, backfill_models ends up empty, and
_plan() raises. The workaround was adding some unrelated active model to the
selection, which works but it is not great.

The thing is, the underlying machinery already handles deletions fine. When
backfill_models has at least one entry, deleted models show up correctly in
context_diff.removed_snapshots. So the fix is about making the selector and
plan validation aware that "no local matches" can still be valid when matching
models pending deletion in the deployed environment.

Changes:

• Extracted _load_env_models() from select_models() to avoid duplicating
  the environment loading logic
• Added expand_model_selections_with_env() that expands selections against
  both local and environment models
• In _plan(), when backfill_models is empty, it now checks whether the
  selection matched models in the environment that were deleted locally. If so,
  PlanError is suppressed and the deletions surface via removed_snapshots
• Selecting a model that exists neither locally nor in the environment still
  raises PlanError as before

Test Plan

• test_expand_model_selections_with_env: deleted models found in env, local
  models still work, wildcards match, non-existent returns empty
• test_expand_model_selections_with_env_fallback: fallback environment is used
  when target env is missing
• test_expand_model_selections_with_env_expired: expired environments ignored
• test_plan_select_model_deleted_model: end-to-end with the sushi example,
  deploys to prod, deletes model file, reloads, runs plan and checks that the
  model appears in removed_snapshots
• test_plan_select_model_deleted_model_still_rejects_nonexistent: negative
  case, makes sure garbage input still fails

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO