[codex] add coverage tests

[senamakel]

Summary

• add coverage for scalar-input approval resume behavior
• add coverage for journaled durable checkpoint resume observations
• add a CLI smoke test for the tinyflows binary output

Validation

• cargo fmt --check
• cargo test
• cargo test --features mock
• cargo llvm-cov --features mock --summary-only --fail-under-lines 90

Coverage with the mock feature is 99.51% line coverage.

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between e70310c and 0601ff3.

📒 Files selected for processing (2)

• src/engine.rs
• tests/cli_e2e.rs

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