Fix merge-gatekeeper: treat cancelled checks as pending; add fail-fast: false to python-tests

- In merge-gatekeeper.yml: treat `cancelled` check conclusions as `pending`
  instead of `error`. A cancelled job means it was interrupted (e.g. by a
  manual cancel or fail-fast cascade), not that it actually failed. The
  gatekeeper now waits for a re-run rather than immediately blocking the PR.
- In python-tests.yml: change `fail-fast: false` so that when one matrix job
  fails, the remaining jobs run to completion instead of being cancelled.
  This prevents cascade `cancelled` conclusions that would trip the gatekeeper.

.github/workflows/merge-gatekeeper.yml

@@ -69,8 +69,14 @@ jobs:
                   continue; // Skipped runs are dropped, matching the original action.
                 } else if (r.conclusion === 'success' || r.conclusion === 'neutral') {
                   state = 'success';
+                } else if (r.conclusion === 'cancelled') {
+                  // A cancelled run means the job was interrupted (e.g. due to
+                  // fail-fast or a manual cancel), not that it actually failed.
+                  // Treat it as pending so the gatekeeper waits for a re-run
+                  // rather than blocking the PR immediately.
+                  state = 'pending';
                 } else {
-                  // cancelled | timed_out | action_required | stale | failure
+                  // timed_out | action_required | stale | failure
                   state = 'error';
                 }
                 merged.set(r.name, { name: r.name, state });

.github/workflows/python-tests.yml

@@ -14,7 +14,7 @@ jobs:
     name: Python Tests
     runs-on: ${{ matrix.os }}
     strategy:
-      fail-fast: true
+      fail-fast: false
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
         # todo: add macos-latest when problems are resolved