docs: add test directory convention to AGENTS.md

[delock]

This PR is to tell agents that new tests needs to go to tests/unit/v1 instead of tests/unit/ to enter CI test.

[chatgpt-codex-connector]

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[tohtana]

Hi @delock,
This update looks good, but isn't "New tests must go under tests/unit/v1/" too strict?
We have the full nightly test. We sometimes miss issues that only the full test capure when we merge a PR, but it is still good to add some non-critical tests there rather than tests/unit/v1/.

The other direction is we can actually add everything to tests/unit/v1/ and occasionally move them to the nightly test.

[delock]

Hi @delock, This update looks good, but isn't "New tests must go under tests/unit/v1/" too strict? We have the full nightly test. We sometimes miss issues that only the full test capure when we merge a PR, but it is still good to add some non-critical tests there rather than tests/unit/v1/.

The other direction is we can actually add everything to tests/unit/v1/ and occasionally move them to the nightly test.

Hi @tohtana thanks for the comments. I agree that we'd expect an agent to figure this out by making a more rational decison based on the situation, rather than following a simple rule.

Let me close this PR and go back if we have better hints.