Add $ping tests

[PatersonProjects]

This PR adds the tests for the $ping operator, following the format of other diagnostic tests

Ref: Issue #43, #444

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort M · Status Needs Review
Confidence: 0.78 (mixed)

Reasoning

component from path globs (test-coverage); effort from diff stats (214+0 LOC, 5 files); LLM: Adds new test cases for the $ping operator following existing diagnostic test patterns, a focused test-coverage addition with minimal LOC impact.

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[fanyangv]

For consistency with buildInfo and listCommands tests, consider adding a test for ping on a nonexistent database (e.g., collection.database.client["nonexistent_db"]["coll"]).

[fanyangv]

For consistency with buildInfo and listCommands error tests, consider adding a test that {"aggregate": "test", "pipeline": [{"$ping": {}}], "cursor": {}} returns UNKNOWN_PIPELINE_STAGE_ERROR.