Run container tests in scheduler context

[samuel-williams-shopify]

Summary

• Use Sus::Fixtures::Async::SchedulerContext for Falcon tests that start Async::Container directly.
• Covers serve command integration and server service container startup paths for async-container v0.36.0.

Testing

• PATH=/Users/samuel/.local/state/tec/toolchain/base_profile/bin:$PATH bundle exec sus --verbose test/falcon/command/serve.rb test/falcon/service/server.rb
• PATH=/Users/samuel/.local/state/tec/toolchain/base_profile/bin:$PATH bundle exec sus --verbose