ci: cap nx parallelism to 3 to fix flaky Test job

nx.json sets `parallel: 5`, which is fine for local dev machines but
oversubscribes GitHub-hosted runners: each affected `test:lib` (vitest) and
`test:types` (tsc) task spawns its own worker pool, so 5 heavy tasks at once
exhaust CPU/RAM and intermittently stall or OOM-kill deterministic tasks. That
produced flaky failures across unrelated PRs (#471, #477, #478) that pass on a
plain re-run.

The team already pins `test:e2e` to `--parallel=1` for the same reason; cap the
Test job's nx parallelism via NX_PARALLEL=3 (CI-only, leaves local dev at 5).
Combined with the raised vitest timeouts, this keeps tasks within the runner's
budget.

Author: AlemTuzlak

.github/workflows/pr.yml

@@ -9,6 +9,12 @@ concurrency:
 
 env:
   NX_CLOUD_ACCESS_TOKEN: ${{ secrets.NX_CLOUD_ACCESS_TOKEN }}
+  # nx.json sets parallel: 5 for local dev machines. GitHub-hosted runners have
+  # far fewer cores/RAM, and each test:lib (vitest) / test:types (tsc) task
+  # spawns its own worker pool. Running 5 such heavy tasks at once oversubscribes
+  # the runner and intermittently OOM-kills or stalls tasks, producing flaky
+  # failures that pass on re-run. Cap CI parallelism to keep tasks within budget.
+  NX_PARALLEL: 3
 
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