Baseline the Inputs.table empty header on the OJS data sources page

[cwickham]

The weekly axe report shows one new finding: empty-table-header on docs/interactive/ojs/data-sources.html. It was also the only unmatched non-image finding in the 2026-10-02 run.

It is the known Observable Inputs problem. The Inputs.table select-all header cell holds only an unlabeled checkbox. The existing empty-table-header :: th entry already lists this page. But on this page axe anchors the selector to the table's id (#oi-3a86ea-1 > table > thead > tr > th:nth-child(1)), so the signature is empty-table-header :: #oi-3a86ea > table > thead > tr > th and does not match.

This PR adds an entry for that signature, scoped to the one page. The 3a86ea namespace is fixed in Inputs, and the signature already drops the -1 counter, so the signature is stable from run to run. When quarto-dev/quarto-cli#14934 updates Inputs, the namespace becomes inputs-3a86ea, and this entry must be replaced (same as the Inputs.range entries).

Runs: 2026-10-05, 2026-10-02.

This change will also need to go to prerelease.

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Successfully created backport PR for prerelease:

• [Sync to prerelease] Baseline the Inputs.table empty header on the OJS data sources page #2269