Abort solidus:install on migration/seed rake failures

[wakqasahmed]

Fixes #6035

Problem

solidus:install runs several rake tasks (railties:install:migrations, db:create, db:migrate, active_storage:install, db:seed, spree_sample:load) via the generator's rake helper without passing abort_on_failure: true.

Even though Solidus::InstallGenerator.exit_on_failure? returns true, Thor's run only falls back to that class-level default when the :abort_on_failure key is absent from the options hash (config.fetch(:abort_on_failure, self.class.exit_on_failure?)). Rails' execute_command always includes the key explicitly (as nil when not set), so the fallback never kicks in and a failing rake task is silently ignored. The generator then proceeds through the rest of its steps and prints Solidus has been installed successfully. Enjoy! even though migrations or seeding actually failed - exactly what was reported in #6035 (a db:migrate failure due to a pre-existing table went unnoticed, seeding then failed too, and the install was reported as successful).

Fix

Pass abort_on_failure: true explicitly on each of these rake calls in core/lib/generators/solidus/install/install_generator.rb, so a real failure aborts the generator (a clean Thor::Error, since exit_on_failure? is already true) instead of continuing silently.

Test plan

• Added specs in core/spec/lib/generators/solidus/install/install_generator_spec.rb asserting abort_on_failure: true is passed for the migrations, database creation, active storage, seed, and sample-data rake calls, and that a raised failure from db:migrate propagates out of run_migrations instead of being swallowed.
• Could not execute the suite locally (no Ruby/Bundler available in this environment) - verified the fix against Rails' Rails::Generators::Actions#rake/#execute_command and Thor's Actions#run source to confirm the exact fallback behavior described above.