CI: do not fail-fast the platform matrix

A failure on one platform cancelled the other two, and the cancellation is
what surfaced, so the reported error was 'The operation was canceled' on a
job that was merely a bystander.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Saloed

.github/workflows/build-and-run-tests.yml

@@ -14,6 +14,9 @@ jobs:
   run_tests:
     name: Run tests
     strategy:
+      # Without this a failure on one platform cancels the others, and the
+      # cancellation is what gets reported, hiding the actual failure.
+      fail-fast: false
       matrix:
         os: [ ubuntu-22.04, windows-latest, macos-latest ]