fix(cli): keep dag validate success output on stdout

[OnlyYu1996]

Summary

• route the normal dag validate success line through the stdout success helper
• keep verbose execution order on stdout, so dag validate --verbose no longer splits normal success output across stdout and stderr
• fix a stale agent command test import that blocks cortex-cli tests on current main

Fixes PlatformNetwork/bounty-challenge#53108.

Validation

• cargo +stable fmt --package cortex-cli --check
• cargo +stable test --package cortex-cli dag_cmd::tests
• cargo +stable check --package cortex-cli --lib
• direct target/debug/Cortex.exe dag validate --verbose --file <dag.json> 1>stdout 2>stderr smoke: success line and execution order in stdout, stderr empty
• git diff --check

[greptile-apps]

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