ci: keep Unit Tests Complete reporting on docs-only pull requests

[Maffooch]

[sc-14625]

Description

Fixes a same-day CI regression from #15699. That change added paths-ignore: ['docs/**'] to the pull_request trigger of unit-tests.yml, so a docs-only pull request never triggers the workflow at all. The required Unit Tests Complete context is then never created and sits as "Expected -- waiting for status to be reported" forever.

The original reasoning was that the merge queue's unfiltered merge_group run would still produce the check. The hole: a pull request must pass its required status checks before it can be added to the merge queue, and the Merge Queue ruleset's required check surfaces on every PR into bugfix. A check that never reports blocks queue entry, so docs-only PRs became unqueueable for anyone without ruleset bypass. Visible today on #15710 and #15711, the first docs-only PRs opened after the change (docs-only PRs merged earlier, #15703/#15704, predate it and ran the full suite at PR level).

What changed

Keeps the CI savings, moves the skip from the trigger to the jobs:

• The pull_request trigger has no path filter again, so the workflow (and the gate context) runs on every PR.
• A new changes job lists the PR's changed files via gh api and outputs docs_only=true only when every file is under docs/. It fails open: any error means docs_only=false and the full suite runs. On merge_group/workflow_dispatch the detection is skipped entirely, so the queue always tests the complete two-platform matrix.
• build-docker-containers skips when docs_only is true; the test-* jobs skip with it through their needs.
• The unit-tests-complete gate accepts a skipped dependency only when docs_only is true, so on a docs-only PR it reports success in seconds instead of never, and a skipped job on any other PR still fails the gate.

A mixed PR (docs + code) has docs_only=false and runs the full light-tier suite, unchanged from before #15699.

Verification

• actionlint passes on the workflow.
• This PR touches .github/, so it exercises the docs_only=false path itself: the full light-tier suite must run and Unit Tests Complete must report green here.
• After this lands, close/reopen (or push to) docs(mfa): disabling MFA now requires a verification code #15710/docs(sensei): document AI Model Settings and its LLM providers #15711 so their pull_request event picks up the fixed workflow from the merge ref; the heavy jobs should show as skipped and Unit Tests Complete should report success within about a minute.
• The next queued PR's merge_group run should still execute the full matrix.

Documentation-only change to CI configuration; no application code changes.

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