Improve feature test coverage parity

[jooh]

Summary

• add upstream-compatible Feature option environment variable normalization shared by config resolution and features test
• cover numeric-leading option names so they map like upstream getSafeId
• add features test coverage for global-only discovery, feature/filter selection, and remote-user execution

Verification

• cargo test --manifest-path cmd/devcontainer/Cargo.toml feature_option
• cargo test --manifest-path cmd/devcontainer/Cargo.toml feature_test_option
• cargo test --manifest-path cmd/devcontainer/Cargo.toml feature_tests
• make tests