Add 92 smoke tests, no_parallel marker, and two-phase parallel handling: window operators, query-and-write, query-planning, sessions, administration, diagnostic, security (auth/roles/users)

[fanyangv]

Closes #60

• 92 smoke tests for: window operators (28), query-and-write commands (6), query planning (4), sessions (5), system administration (13), diagnostics (18), security (18)
• Add no_parallel pytest marker for 14 tests that modify global server state (kill sessions/ops, set parameters, drop all users/roles)
• Add pytest_collection_modifyitems hook to auto-deselect no_parallel tests under xdist
• Add pytest_sessionfinish hook to automatically run no_parallel tests sequentially after parallel phase
• Add worker_id fallback fixture for running without xdist
• Update README with two-phase execution docs

[eerxuan]

Verified locally: two-phase parallel handling works correctly

• Phase 1 (parallel, -n auto): 674 tests passed in 20.75s — no_parallel tests deselected
• Phase 2 (sequential, automatic): 14 no_parallel tests passed in 5.12s

One issue found:

⚠️ Failed to merge JSON reports: [Errno 2] No such file or directory: '.report.json'

When --json-report is not explicitly passed, the json_report_file option still has a default value (.report.json) from the plugin. Phase 2 sees this default, generates a temp report, and tries to merge into a file that Phase 1 never wrote.

Fix: check if Phase 1 report actually exists before merging. If it doesn't exist and Phase 2's does, either copy Phase 2's report as the final report or skip silently (no warning):

if phase1_json and phase2_json and os.path.exists(phase2_json):
    if os.path.exists(phase1_json):
        _merge_json_reports(phase1_json, phase2_json)
    else:
        # Phase 1 didn't write a report (--json-report not passed)
        # Skip merge silently — Phase 2 temp file will be cleaned up
        pass

Same applies to JUnit XML merge.

[eerxuan]

Minor: tempfile.mktemp is deprecated (https://docs.python.org/3/library/tempfile.html#tempfile.mktemp). Consider tempfile.NamedTemporaryFile(delete=False, suffix=".json", prefix="phase2_") instead. Not a practical concern here, just a lint/best-practice thing.