ci: limit test threads to 8 to reduce flakiness

High-core-count runners (64 CPUs) run tests with too much parallelism,
causing timing-sensitive network tests to fail intermittently.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sanity

.github/workflows/ci.yml

@@ -127,7 +127,9 @@ jobs:
           rm -rf /tmp/freenet /tmp/freenet-* 2>/dev/null || true
 
       - name: Test
-        run: cargo test --workspace --no-default-features --features trace,websocket,redb
+        # Limit test threads to reduce resource contention on high-core-count runners
+        # Without this, 64+ parallel tests cause timing-sensitive network tests to fail
+        run: cargo test --workspace --no-default-features --features trace,websocket,redb -- --test-threads=8
 
   six_peer_regression:
     name: six-peer-regression