[Maffooch]

Shortcut: sc-16697

Summary

The docs-only skip in unit-tests.yml (#15699, #15712) only looked at pull_request events. On merge_group the detection step never ran, so every queued docs PR rebuilt both platforms and ran the full suite before landing.

This extends the changes job to the merge queue:

• On merge_group, it diffs merge_group.base_sha against merge_group.head_sha. The head commit includes every entry queued ahead, so the heavy jobs skip only when that whole union touches nothing outside docs/. The code under test is then identical to the base, which already passed through the queue.
• The diff uses two depth-1 git fetches instead of the compare API, which truncates at 300 files.
• Parser doc pages (docs/content/supported_tools/parsers/) no longer count as docs-only on either event. unittests/test_parsers.py checks that every parser has a doc page and every doc page has a parser, so renaming or deleting one can break the suite.
• It still fails open: if the listing fails, docs_only stays false and the full suite runs. Unit Tests Complete accepts a skipped dependency only when docs_only is true.

I ran the step's script locally against real dev commits: a docs-only commit (#16406) gives true, a code fix (#16409) gives false, and bogus SHAs give false with exit 0. actionlint is clean. The real merge_group path can only be exercised once this is on dev.

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