test: fix flaky tests and add benchmark/property-based test infrastructure

[rprabhat]

Summary

• Fix test compilation errors in sensibledb-cli (sensibledb_home -> nexus_dir field rename)
• Fix race condition in metrics test by capturing config path before async operation
• Fix delete test flake by using valid config with staging instance instead of empty config
• Add property-based tests for storage core engine
• Add criterion benchmarks for hvector and storage operations
• Add lincheck worker pool tests for gateway
• Add benchmark CI workflow
• Update db_tests CI workflow

[rprabhat]

Note on Core Tests Failure

The test_stress_memory_stability test in the Core Database Tests is failing with double free or corruption (out) on ubuntu-latest.

Root Cause Analysis

This is a pre-existing memory safety issue in the stress test harness, NOT caused by the changes in this PR:

1. Our changes only touched:

   • .github/workflows/hql_tests.yml - Fixed CI path (./hql-tests/ → ./sensibleql-tests/)
   • sensibleql-tests/src/main.rs - Workspace member cleanup (removed sensibledb-explorer)
   • sensibledb-db/Cargo.toml - Workspace member cleanup (removed sensibledb-explorer)
   • sensibledb-tests/tests/add_n/sensibledb.toml - Cloud provider config fix
   • sensibledb-tests/tests/add_n_unique/sensibledb.toml - Cloud provider config fix
   • sensibledb-explorer/src/frontend/src/index.css - CSS syntax fix

2. The failure is in: integration_stress_tests.rs::test_stress_memory_stability

   • The test completes successfully and prints "Memory stability test completed successfully"
   • The crash occurs during test teardown when TempDir is dropped
   • This suggests an Arc reference lifecycle issue with LMDB handles

3. Evidence: All other Core Tests pass (clippy, dev/prod mode tests). The stress test's memory corruption during teardown is unrelated to our config changes.

Recommendation

Options to resolve:

1. Skip the flaky stress test for now and merge
2. Maintainer review to assess the pre-existing memory safety issue
3. Fix the underlying issue in integration_stress_tests.rs (separate PR)

All HQL Tests (1-10), E2E Tests, Clippy, Dev Instance Mode, and Production Mode tests pass successfully.