test: add failing test for multiple colliders bug (#878)

[ANAMASGARD]

FIXES: #878

Reproduces the error when check_dag encounters multiple colliders in adjustment nodes, causing: 'length = 2' in coercion to 'logical(1)'

The test currently fails as expected and will pass once the bug is fixed in the .finalize_dag() function.

What This PR Does

• Adds a test case that reproduces the exact error from #878
• Uses expect_error() to verify the bug exists
• Test will FAIL in CI (expected behavior)
• Demonstrates the bug before applying the fix

Next Steps

After this PR is reviewed/merged, I will submit a follow-up PR with:

1. The actual bug fix (one-line change)
2. Updated test to verify the fix works

[gemini-code-assist[bot]]

Summary of Changes

Hello @ANAMASGARD, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new failing test case to precisely reproduce a bug identified in issue #878. The test demonstrates an error occurring in the check_dag function when it processes multiple colliders within adjustment nodes. This PR serves as a preliminary step, establishing a clear reproduction of the bug before a separate follow-up PR will provide the actual fix and update the test to pass.

Highlights

• New Failing Test Case: Introduces a new test_that block in test-check_dag.R to reproduce the bug described in issue #878.
• Bug Reproduction: The test specifically targets the error where check_dag encounters multiple colliders in adjustment nodes, leading to an error message "length = 2" in coercion to "logical(1)".
• Expected Failure: The added test is designed to fail currently, confirming the presence of the bug, and is expected to pass once the fix is applied in a subsequent pull request.

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