Add $bucketAuto _id override and reshaped-upstream compatibility tests

[jingyaog]

Summary

• Add _id-in-output override tests for $bucketAuto (accumulator overrides the
  default {min, max} boundary; non-accumulator _id is rejected with 40234) —
  contrasting $bucket, which reserves _id.
• Add reshaped/re-materialized-upstream tests: $bucketAuto after $unwind,
  $project, $addFields, $unset, $replaceRoot (plus a pass-through $match
  control), with wide docs and buckets > distinct groupBy values.
• Generalize both cases as coverage guidelines in TEST_COVERAGE.md §15.

Test plan

All new tests pass against the reference server (MongoDB 8.2.4, the target spec version).

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, documentation, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort M · Status Needs Review
Confidence: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, docs); effort from diff stats (204+1 LOC, 4 files); LLM: Adds new compatibility test cases for $bucketAuto edge cases and updates TEST_COVERAGE.md, touching test files and docs within a single component.

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