Stop assigning validation_context in sane_ancestor_ids?

[SAY-5]

Description

Rails main removed the private validation_context= writer (rails/rails#58220), so sane_ancestor_ids? now raises NoMethodError whenever a node with descendants is moved or check_ancestry_integrity! runs. The context juggling was only there to give a cleaner error and nothing in the method reads it (the validators are invoked directly through validate_each), so this drops it rather than reaching into the new internal ValidationContext object. Fixes #766

Before

child.update!(parent: nil) on Rails main: NoMethodError: undefined method 'validation_context=' for an instance of Node

After

Move and integrity check work on Rails main, behavior unchanged on released Rails.

Type of Change

• Bug fix

Checklist

• My code follows the style guidelines (e.g., RuboCop)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally (bundle exec rake test)

How Has This Been Tested?

DB=sqlite3 bundle exec rake test on the default Gemfile, 258 runs 0 failures. The existing sane_ancestor_ids? tests in the materialized path suites cover the method; Rails main is not in the appraisal matrix so the failing path itself is not exercised in CI.