Add fsync command tests

[danielfrankcom]

This change adds tests for the fsync command.

There is some machinery overlap with #641 but they have the same content so they should merge cleanly.

Closes #420

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort L · Status Needs Review
Confidence: 0.30 (deterministic)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (1081+1 LOC, 11 files); LLM failed: Invalid response body while trying to fetch https://api.anthropic.com/v1/messages: Premature close

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[ramgokul-14]

Required - Doc update needed for conftest.py validator allowance — the change to test_structure_validator.py is a project-wide convention change. Per REVIEW.md §9, update TEST_FORMAT.md or FOLDER_STRUCTURE.md to document that conftest.py is a valid non-test Python file.

Suggestions

• Add a max-iteration guard to drain_fsync_lock's while True to prevent CI hangs
• Assert the first lock acquisition succeeded in test_fsync_lock_persists_after_connection_close before closing the client
• Consider an inline comment in test_fsync_rejects_non_admin_database explaining why execute_command (non-admin) is used

Questions

• Is afterClusterTime in readConcern in-scope, or is the current level-focused testing sufficient for an admin command that mostly rejects these options?
• Was placing the fixture at administration/conftest.py (parent level) considered vs. the re-export pattern?