Add 'smoke' category and parallelism to tests

[joseharriaga]

From @trrwilson:

• An informal "smoke test" category is added and applied to tests that are extremely fast and require no auth/credentials -- perfect for ensuring client creation, serialization, etc. work
• A couple of new mock chat tests -- including one that would have caught last weekend's regression -- are included in this category
• The "smoke" category is added to the CI workflow; this only adds 2-3s to the test task
• NUnit Parallelization is enabled (degree: 8) to speed up live test runs; this currently reduces a ~10m run to ~1.5m
  • As written, we're bottlenecked by Assistants and VectorStore tests, as they exercise several stateful assumptions and have to be kept serial with respect to themselves; this is likely unnecessarily fragile to begin with and can be fixed with rewrites later
  • I didn't encounter any rate limit issues, but it's easy to adjust the degree of parallelism and/or per-fixture parallelization as needed