Add $bucketAuto aggregation stage compatibility tests

[jingyaog]

Feature being tested

The $bucketAuto aggregation stage — automatic bucketing of documents into a
specified number of buckets based on a groupBy expression, with even document
distribution, boundary/type preservation, optional preferred-number granularity
series, and per-bucket output accumulators.

Test cases

139 test cases across 10 files:

File	Cases	Area
bucketAuto/test_bucketAuto_core_semantics.py	8	Empty/nonexistent collection, single doc, identical values, fewer unique/docs than buckets, even/uneven distribution
bucketAuto/test_bucketAuto_boundaries.py	16	Boundary shape (min inclusive, interior max exclusive, final max inclusive, shared edges, global bounds), BSON type preservation, numeric equivalence, mixed-type ordering, Decimal128 precision (incl. extreme min/max)
bucketAuto/test_bucketAuto_buckets_validation.py	25	buckets accepts int32/int64/whole-double/whole-Decimal128/1; rejects 0/negative/fractional/NaN/Infinity/overflow/non-numeric types
bucketAuto/test_bucketAuto_groupby.py	7	groupBy expression types, null/missing grouping
bucketAuto/test_bucketAuto_arg_errors.py	24	Argument container validation (non-object types, unrecognized/case-variant options, missing required)
bucketAuto/test_bucketAuto_granularity.py	18	All preferred-number series accepted; R5 / 1-2-5 / E6 / POWERSOF2 rounding; coarse→fewer buckets; unknown/empty/non-string rejection; non-numeric/NaN/negative groupBy rejection
bucketAuto/test_bucketAuto_output.py	12	Implicit count, output replaces count, explicit {$sum:1}, empty output, multiple accumulators, input-vs-sibling refs, nested expressions, $push with $$ROOT/$$CURRENT/$$REMOVE, special/duplicate field names
bucketAuto/test_bucketAuto_output_errors.py	20	Output non-object types, $-prefix, dotted names, non-accumulator values
bucketAuto/test_bucketAuto_accumulator_errors.py	2	Accumulator error propagation (field-driven conversion, literal divide-by-zero)
stages/test_stages_position_bucketAuto.py	7	Multi-stage interactions ($match before; $sort/$limit/$skip after; $facet; feeding $group/$project) — placed in parent stages/ folder per FOLDER_STRUCTURE.md

Framework additions:

• error_codes.py — $bucketAuto error-code constants (40239–40246, 40257–40261)
• test_constants.py — GRANULARITY_VALUES preferred-number series list

MongoDB 8.2 documentation

• https://www.mongodb.com/docs/manual/reference/operator/aggregation/bucketAuto/
• https://www.mongodb.com/docs/manual/reference/operator/aggregation/group/ (accumulators used in output)

TEST_COVERAGE.md guideline updates

None. No changes to TEST_COVERAGE.md were required — the existing §15 (pipeline
stages) and §18 (accumulators, which already names $bucketAuto) already scope this
feature. The new error-code constants and GRANULARITY_VALUES are additive framework
support and did not warrant new rule text.

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort XL · Status Needs Review
Confidence: 0.90 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (2139+0 LOC, 12 files); LLM: Adds 141 new compatibility test cases across 10 files covering the $bucketAuto aggregation stage, plus framework-level additions to error_codes.py and test_constants.py, touching multiple files across the test-coverage component.

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[eerxuan]

bucketAuto specific, put under /bucketAuto/utils/