test(csharp): add long-running (hours-long) query E2E tests scheduled nightly

Problem

We have no CI coverage for genuinely long-running (hours-long) queries. The doc calls for such tests scheduled nightly, not per-commit.

The repo has exactly one scheduled workflow: .github/workflows/reyden-rest-nightly.yml (cron '0 2 * * *', lines 17-21). Everything else is per-PR/push/dispatch.
The longest test today, StatusPollerKeepsQueryAlive (csharp/test/E2E/StatementTests.cs:949-975), is a real 30-minute test (Task.Delay(30 min) over RANGE(30000000)) — but it is excluded from every CI run: Skip.If(TestConfiguration.IsCITesting, …) (line 955), and both the nightly (reyden-rest-nightly.yml:164) and per-PR e2e (e2e-tests.yml:232) set isCITesting: true. So no long query ever executes in CI.
There is no test-category convention — zero [Trait]/[Category] attributes in csharp/test. CI runs dotnet test with no --filter (ci/scripts/csharp_test_databricks_e2e.sh).
The nightly failure-classification/dashboard machinery (ci: classify Reyden nightly E2E failures by root cause #512/feat(ci): split Reyden gap signatures + per-error dashboard detail #513) is staged on branch ci/reyden-rest-nightly, not yet on main — a new long-running tier should coordinate with it.

Introduce a long-running test category — either an xUnit [Trait("Category","LongRunning")] driven by dotnet test --filter, or a dedicated config flag analogous to IsCITesting (note: the existing Skip.If(IsCITesting) on StatusPollerKeepsQueryAlive must be inverted/relaxed so it can run in the long-running tier).
Add hours-long query tests targeting existing large data (e.g. main.tpcds_sf100_delta.*, already used by DriverResilienceTests.cs:98-135; larger scale factors available).
Wire them into reyden-rest-nightly.yml (or a sibling scheduled workflow).

GitHub-hosted runners have a 6-hour job limit; reyden-rest-nightly uses concurrency … cancel-in-progress: false.
dotnet test currently has no timeout override — hours-long tests need explicit per-test timeout handling.