fix(ci): run tests sequentially to avoid shared SpacetimeDB state conflicts

SpacetimeDB integration tests share a single server instance. When tests
run in parallel (default behavior), concurrent delete_all() calls from
different tests interfere with each other. Use --test-threads=1 to
serialize all integration tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: konard

.github/workflows/rust-benchmark.yml

@@ -125,7 +125,8 @@ jobs:
         env:
           SPACETIMEDB_URI: http://localhost:3000
           SPACETIMEDB_DB: benchmark-links
-        run: cargo test
+        # Run tests sequentially to avoid parallel interference with shared SpacetimeDB state.
+        run: cargo test -- --test-threads=1
 
   benchmark:
     name: Benchmark