refactor(tests): strip mongomock_ng references from all test files

[engFelipeMonteiro]

Strip all mongomock_ng string references from test files except the alias import mongomock_ng as mongomock.\n\nChanges across 10 files (40+ individual edits):\n- Import rewrite: from mongomock_ng import X \u2192 from mongomock import X\n- Submodule paths: from mongomock_ng.collection import \u2192 from mongomock.collection import\n- Variable rename: mongomock_ng_utcnow \u2192 mongomock_utcnow (test__collection_api.py)\n- Test-db strings: mongomock_ng___testing_db \u2192 mongomock___testing_db (test__gridfs.py, test__bulk_operations.py)\n- Comment URL: update vmalloc/mongomock_ng \u2192 vmalloc/mongomock\n- conftest.py kept unchanged (needs module object for sys.modules trick)\n\nAdds tests/test__import_consistency.py with AST-based verification that no mongomock_ng strings leak past imports in any test file.\n\nRationale: clean diffs when porting upstream mongomock PRs. Only 1 line (import mongomock_ng as mongomock) differs from upstream per file.\n\nFiles changed: 11 (10 edits + 1 new test file)

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