Add compatibility tests for $percentile expression operator

[Kwadwot]

What does this PR do?

Adds comprehensive compatibility tests for the $percentile aggregation expression operator (used inside $project/$set/$addFields), which previously had only a single happy-path smoke test. The new tests live under expressions/accumulator/percentile/ and follow the $avg sibling pattern: a PercentileTest(BaseTestCase) dataclass plus a percentile_spec() builder in utils/, the shared execute_expression helpers, and assert_expression_result. Coverage is split by aspect across 7 files (49 cases): core selection and p ordering, input forms, numeric types and special values, non-numeric handling, null/missing input, p validation, and method/spec validation. Two error-code constants used by the tests are added to framework/error_codes.py: PERCENTILE_INVALID_P_TYPE_ERROR (7750302) and PERCENTILE_SPEC_NOT_OBJECT_ERROR (7436200).

Why was this PR needed?

$percentile is a tracked feature in the taxonomy (docs/feature-tree.csv) but had only smoke-level coverage. Characterizing its behavior against a live server surfaced specifics worth pinning down: under method: "approximate" it returns an order statistic at rank ceil(p·n) and always returns double (even for Decimal128 input); non-numeric/null/missing inputs yield [null]; and malformed p/method/spec values map to distinct error codes. Two findings emerged during investigation (captured on MongoDB 8.3.4): method: "discrete" and "continuous" are not supported (only "approximate", others return BadValue), and passing NaN as a p value crashed the mongod process — that case was deliberately excluded from the suite as a likely upstream robustness bug.

What are the relevant issue numbers?

Closes #195

How to test it (on a MongoDB instance) + Screenshot:

Test run against MongoDB 8.3.4:

pytest documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/ --connection-string mongodb://localhost:27017 --engine-name mongodb -q

49 passed in 1.18s

Tests success screenshot:

Regression check on the reference sibling suite ($avg): 224 passed.

Does this PR meet the acceptance criteria?

• Tests added for new/changed behavior — 49 cases across 7 files
• All tests passing — 49 passed; $avg regression 224 passed; isort/flake8/mypy clean
• Follows project style guide — mirrors the $avg pattern per TEST_FORMAT.md / TEST_COVERAGE.md / FOLDER_STRUCTURE.md
• No breaking changes introduced — additive only (new tests + two sorted error-code constants)
• Documentation updated (if applicable) — n/a (test-only contribution; no user-facing docs affected)

[Copilot]

Pull request overview

Adds comprehensive compatibility coverage for the $percentile aggregation expression operator, aligning the test structure with existing accumulator-expression suites (e.g., $avg) and codifying observed MongoDB 8.3.4 behaviors and error-code mappings.

Changes:

• Adds a new $percentile compatibility test suite with focused coverage across core semantics, input forms, numeric types/special values, null/missing handling, non-numeric handling, p validation, and method/spec validation.
• Introduces a shared PercentileTest dataclass and percentile_spec() helper to keep test case construction consistent and concise.
• Adds two new error-code constants used by the new validation tests.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
documentdb_tests/framework/error_codes.py	Adds two $percentile-related error code constants used by the new tests.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/utils/percentile_common.py	Introduces shared PercentileTest and a helper to build $percentile specs.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_core.py	Covers core selection semantics and output ordering relative to p.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_input_forms.py	Verifies supported input expression forms and empty-input behavior.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_data_types.py	Validates numeric-type handling and special-value ordering/behavior.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_non_numeric.py	Ensures non-numeric filtering and all-non-numeric null-result behavior.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_null.py	Covers null and missing-input behavior (including field-reference cases).
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_p_validation.py	Adds p shape/range/type validation cases and expected error-code assertions.
documentdb_tests/compatibility/tests/core/operator/expressions/accumulator/percentile/test_percentile_method.py	Validates method acceptance/rejection and spec-document validation/error codes.

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement, performance
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 (623+0 LOC, 9 files); LLM failed: Invalid response body while trying to fetch https://api.anthropic.com/v1/messages: Premature close

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