ci: drop Node 26 from test matrix (#3695)

Node 26 lanes have been causing recurring instability and CI churn.
Drop the Node 26 entry from the strategy matrix in .github/workflows/test.yml
until the Node 26 release stabilises. Also remove the companion
continue-on-error guard and its explanatory comment block, which
existed solely to prevent Node 26 failures from gating the workflow.

Files changed:
  .github/workflows/test.yml — removed node-version: 26 from matrix,
    removed continue-on-error: ${{ matrix.node-version == 26 }} and its
    two-line comment block

Preserved: node-version: [22, 24] still run on all three OS lanes
  (ubuntu-latest, macos-latest, windows-latest).

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>