Add CLI flag coverage tests for yargen and yargen-util

[Neo23x0]

Summary

• merge latest main into codex/tests-1
• add focused tests for single-file CLI flow in yargen (-f behavior and edge cases)
• add submit-flow tests in yargen-util to validate request flags and wait-timeout behavior
• refactor single-file input resolution into a reusable helper for deterministic unit testing

Testing

• go test ./cmd/yargen ./cmd/yargen-util
• go test ./...