fix: restore integration test discovery and sourced bootstrap flow

[philippemnoel]

What

• fix integration rspec glob pattern in rake and script runners
• make scripts/rbenv_bootstrap.sh safe when sourced by using return semantics

Why

• integration runs were not discovering *_integration_spec.rb files
• sourced bootstrap path could exit parent scripts before tests executed

How

• use a single valid integration glob: **/*_integration_spec.rb
• add bootstrap_return helper to return when sourced and exit when executed directly

Tests

• bash -n scripts/rbenv_bootstrap.sh scripts/run_integration_tests.sh
• ruby -e 'puts Dir.glob("spec/**/*_integration_spec.rb").size'