Fix dag validate singular task output

[OnlyYu1996]

Summary

• use singular task in cortex dag validate success output when the DAG has exactly one task
• keep plural tasks for all other counts
• add focused regression coverage for the task-count label
• fix a stale test import so the affected crate tests compile on current main

Fixes PlatformNetwork/bounty-challenge#43808

Tests

• cargo fmt --package cortex-cli --check
• cargo test --package cortex-cli dag_cmd::tests::test_dag_validate_task_count_label_uses_singular_for_one_task -- --nocapture
• cargo test --package cortex-cli dag_cmd::tests -- --nocapture
• cargo check --package cortex-cli --lib
• cargo run --package cortex-cli --bin Cortex -- dag validate --file /tmp/.../single-task.yaml
• git diff --check

[greptile-apps]

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