test: pin the orphan policy contract for solved plans

Component tests for the graph-integrity check from the previous
commit. Test-only; no production code changes.

Six tests in ``tests/component/test_graph_integrity.py``:

- ``test_solve_conda_accepts_pip_via_python_add_pip_dependency``:
  the *normal* shape. ``python.depends == ["pip"]`` makes ``pip``
  forward-reachable; ``pip`` inherits ``python``'s requested
  category without rescue.
- ``test_solve_conda_hard_fails_when_python_metadata_omits_pip``:
  the abnormal-metadata shape. ``python.depends == []`` makes
  ``pip`` unreachable; the orphan check hard-fails. The test
  docstring names this as broken solver metadata, not normal
  solver auto-install behavior.
- ``test_solve_conda_envvar_demotes_orphan_to_warning``: pins
  ``CONDA_LOCK_ALLOW_ORPHANED_LOCKFILE=1`` semantics. WARNING
  fires (explicitly mentioning ``main``,
  ``--dev-dependencies`` / ``-e <category>``, and the envvar
  name so the message can't silently weaken), orphans are
  assigned to ``main``, ``solve_conda`` returns instead of
  raising.
- ``test_solve_conda_hard_fails_on_unrecoverable_orphan``: the
  classic conda#896-corrupt-cache shape -- ``zlib`` planned with
  empty depends, no categorized package transitively requires
  it -- hard-fails with regenerate-from-sources guidance and
  the envvar escape named in the error message.
- ``test_solve_conda_orphan_via_dep_breakage_still_hard_fails``:
  multi-category corrupt-cache variant.
- ``test_solve_conda_passes_when_dependency_graph_is_intact``:
  positive sanity check.

Author: claude