Restore the Active Record connection after building fixtures

[nertzy]

spec:fixture_builder:build establishes the test connection before loading the fixture builder:

ActiveRecord::Base.establish_connection(:test)

It does not restore the caller’s previous connection afterward. A later task in the same Rake process can therefore use the test database even while Rails.env still reports another environment.

This was reproduced on current master with separate development and test SQLite databases. Resetting Rails.env does not repair the connection leak.

• Keep the test connection active while fixtures are being built.
• Restore the caller’s previous connection after a successful build.
• Restore it when fixture generation raises.
• Add regression coverage that identifies the actual database used before, during, and after the build.

This preserves safe task composition without automatically modifying Rails’ standard db:fixtures:load task.