Add test format validation, error codes framework, and $divide tests

Signed-off-by: Yunxuan Shi <yunxuan@amazon.com>

Author: eerxuan

CONTRIBUTING.md

@@ -31,11 +31,19 @@ Thank you for your interest in contributing to the DocumentDB Functional Tests!
 
 ## Writing Tests
 
+For comprehensive testing guidance, see our detailed documentation:
+
+- **[Quick Reference](docs/testing/QUICK_REFERENCE.md)** - Fast lookup for all testing rules
+- **[Test Format Guide](docs/testing/TEST_FORMAT.md)** - Test structure, naming, assertions, and tags
+- **[Test Coverage Guide](docs/testing/TEST_COVERAGE.md)** - Coverage strategies and edge case testing
+- **[Folder Structure Guide](docs/testing/FOLDER_STRUCTURE.md)** - Where to put tests with decision tree
+
 ### Test File Organization
 
 - Place tests in the appropriate directory based on the operation being tested
 - Use descriptive file names: `test_<feature>.py`
 - Group related tests in the same file
+- See [Folder Structure Guide](docs/testing/FOLDER_STRUCTURE.md) for detailed organization rules
 
 ### Test Structure
 
@@ -57,15 +65,23 @@ def test_descriptive_name(collection):
     - Any special conditions or edge cases
     """
     # Arrange - Insert test data
-    collection.insert_one({"name": "Alice", "age": 30})
+    collection.insert_one({"a": 1, "b": 2})
 
-    # Act - Execute the operation being tested
-    result = collection.find({"name": "Alice"})
+    # Execute the operation being tested, use runCommand format
+    execute_command(collection, {"find": collection.name, filter: {"a": 1}})
 
-    # Assert - Verify expected behavior
-    assert len(list(result)) == 1
+    # Assert expected behavior, don't use plain assert for consistent failure log format
+    # Assert whole output when possible, to catch all unexpected regression
+    expected = [{"_id": 0, "a": 1, "b": 2}]
+    assertSuccess(result, expected)
 ```
 
+### Test Case Guidelines
+
+- Each test function defines one test case
+- One assertion per test function
+- Use execute_command for all MongoDB operations
+
 ### Naming Conventions
 
 - **Test functions**: `test_<what_is_being_tested>`
@@ -131,10 +147,10 @@ The framework provides three main fixtures:
        collection.insert_one({"name": "Alice"})
 
        # Act - Execute operation
-       result = collection.find_one({"name": "Alice"})
+       result = execute_command(collection, {"find": collection.name, filter: {"name": "Alice"}})
 
        # Assert - Verify results
-       assert result["name"] == "Alice"
+       assertSuccess(result, {"name": "Alice"})
        # Collection automatically dropped after test
    ```
 

docs/testing/TEST_COVERAGE.md

@@ -0,0 +1,125 @@
+# Test Format Guide
+
+## Test Structure
+
+Every API test follows: Setup → Execute → Assert.
+
+```python
+def test_descriptive_name(collection):
+    """Clear description of what this test validates."""
+    # Setup (insert documents if needed)
+    collection.insert_many([{"_id": 0, "a": 1}, {"_id": 1, "a": 2}])
+    
+    # Execute — always use runCommand format
+    result = execute_command(collection, {
+        "find": collection.name,
+        "filter": {"a": 1}
+    })
+    
+    # Assert — use framework assertion helpers
+    assertSuccess(result, [{"_id": 0, "a": 1}])
+```
+
+## Naming
+
+**Files:** `test_<feature_area>.py` — files in feature subfolders must include the feature name.
+```
+✅ /tests/aggregate/unwind/test_unwind_path.py
+❌ /tests/aggregate/unwind/test_path.py
+```
+
+**Functions:** `test_<what_is_being_tested>` — descriptive, self-documenting.
+```
+✅ test_find_with_gt_operator, test_unwind_preserves_null_arrays
+❌ test_1, test_query, test_edge_case
+```
+
+## Assertions
+
+Use helpers from `framework.assertions`, not plain `assert`:
+
+```python
+# assertResult — parametrized tests mixing success and error cases
+assertResult(result, expected=5)          # checks cursor.firstBatch == [{"result": 5}]
+assertResult(result, error_code=16555)    # checks error code only
+
+# assertSuccess — raw command output
+assertSuccess(result, [{"_id": 0, "a": 1}])
+assertSuccess(result, expected, ignore_order=True)
+
+# assertFailureCode — error cases (only check code, not message)
+assertFailureCode(result, 14)
+```
+
+**One assertion per test function.** Split multiple assertions into separate tests.
+
+## Fixtures
+
+- `collection` — most common. Auto cleanup after test. Insert documents in test body.
+- `database_client` — when you need multiple collections or database-level ops. Auto dropped after test.
+- `engine_client` — raw client access.
+
+## Execute Command
+
+Always use `execute_command()` with runCommand format to get test result, not driver methods. Setups can use methods.
+
+```python
+# ✅ runCommand format
+result = execute_command(collection, {"find": collection.name, "filter": {"a": 1}})
+
+# ❌ Driver methods
+result = collection.find({"a": 1})
+```
+
+## Helper Functions
+
+Avoid deep helper function chains. One layer of abstraction on top of `execute_command()` is acceptable, don't add more abstraction layers unless justified.
+
+```python
+# ✅ Good: execute_expression wraps execute_command with aggregate pipeline boilerplate
+result = execute_expression(collection, {"$add": [1, 2]})
+
+# ❌ Bad: trivial wrappers that just save a few characters add indirection for no clarity gain
+# result = execute_operator(collection, "$add", [1, 2])
+```
+
+Keep helpers in `utils/` at each test level. Helpers should reduce meaningful boilerplate (e.g., building an aggregate pipeline), not just shorten a single line.
+
+Minimize helper scope — one helper should do one thing. If a helper has many if/else branches handling different cases, split it into separate helpers at a lower folder level.
+
+## Parametrized Tests
+
+Use `@pytest.mark.parametrize` with dataclasses for operators with many test cases:
+
+```python
+@dataclass(frozen=True)
+class DivideTest(BaseTestCase):
+    dividend: Any = None
+    divisor: Any = None
+
+DIVIDE_TESTS: list[DivideTest] = [
+    DivideTest("int32", dividend=10, divisor=2, expected=5.0, msg="Should divide int32 values"),
+    DivideTest("null_divisor", dividend=10, divisor=None, expected=None, msg="Should return null when divisor is null"),
+    DivideTest("string_err", dividend=10, divisor="string", error_code=TYPE_MISMATCH_ERROR, msg="Should reject string"),
+]
+
+@pytest.mark.parametrize("test", DIVIDE_TESTS, ids=lambda t: t.id)
+def test_divide(collection, test):
+    """Test $divide operator."""
+    result = execute_expression(collection, {"$divide": [test.dividend, test.divisor]})
+    assertResult(result, expected=test.expected, error_code=test.error_code, msg=test.msg)
+```
+
+- `BaseTestCase` (from `framework.test_case`) provides `id`, `expected`, `error_code`, `msg` — extend it per operator
+- Shared helpers/dataclasses live in `utils/` at each level
+- `msg` is **required** — describes expected behavior, not input
+- Use constants from `framework.test_constants` (`INT32_MAX`, `FLOAT_NAN`, etc.) and `framework.error_codes` (`TYPE_MISMATCH_ERROR`, etc.)
+
+## Validation
+
+A pytest hook auto-validates during collection:
+- Files must match `test_*.py` (except `__init__.py`)
+- Test functions must have docstrings
+- Must use assertion helpers, not plain `assert`
+- One assertion per test function
+- Must use `execute_command()` or helpers from utils