Remove `Session.array_transform` attribute (obsolete single-transducer-pose model)

Summary

Remove the array_transform: ArrayTransform field from openlifu.db.Session, along with its to_dict / from_dict handling, the associated docstring notes, and every reference in examples, tests, and test-resource JSON.

Motivation

Session.array_transform was designed around the assumption that a session has a single canonical "the transducer position." In SlicerOpenLIFU that assumption no longer holds:

• Each page that renders the transducer (pre-planning, localization, solution) picks whichever of the persisted transforms it needs (a specific approved virtual-fit result, a specific approved transducer-tracking result, or -- on the localization page -- both simultaneously).
• The full list of transducer transforms lives in session.virtual_fit_results and session.transducer_tracking_results, keyed by target / photoscan.
• Approval invalidation is now driven per-VF and per-TT rather than by writing back a single pose.

As a result, SlicerOpenLIFU has stopped reading and writing session.array_transform (see the accompanying downstream PR). The field is now unused by the primary consumer and only serves to confuse the persistence model.

Scope of changes required in this repo

• src/openlifu/db/session.py
  • Remove the array_transform field definition on Session.
  • Remove the corresponding to_dict / from_dict handling for array_transform.
  • Update the solution_id field docstring, which currently claims the id is "cleared whenever the array_transform changes." That invalidation policy needs to be reconsidered (probably driven by VF / TT approval changes on the consumer side); at minimum, drop the array_transform reference from the doc.
• examples/tutorials/02_Database_Interaction.py and the matching .ipynb -- drop the array_transform=ArrayTransform(...) argument from the Session(...) construction.
• tests/test_database.py -- remove assertions on session.array_transform.matrix.shape / .units and any loop variables named array_transform that come from the same fixture.
• Test resource JSON files under tests/resources/example_db/ that carry an "array_transform" block -- either drop the block or regenerate the fixtures so they no longer contain it. from_dict should tolerate the field being absent, so old on-disk sessions still load cleanly.
• Sample database (openlifu-sample-database repo) sessions currently carry an "array_transform" block too; that repo will need a matching sweep, but that is out of scope for this issue.

Backwards compatibility

Session.from_dict should silently ignore a legacy "array_transform" key so that existing on-disk sessions (including the pinned openlifu-sample-database fixtures) still load with the new library version.

Downstream

Coordinated with the SlicerOpenLIFU cleanup that decommissions the array_transform read/write path on the consumer side.

Start with src/openlifu/db/session.py and inspect Session serialization, then review the Session construction in examples/tutorials/02_Database_Interaction.py and its matching notebook. Run the database tests in tests/test_database.py and inspect tests/resources/example_db/ for legacy array_transform data. Done means current fixtures and examples no longer reference the field while from_dict still loads legacy sessions.