Add Accumulator Coverage to TEST_COVERAGE.md

[alinaliBQ]

Add Accumulator Test Coverage section to TEST_COVERAGE.md, based on @eerxuan's comment: #214 (review)

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: documentation, enhancement
Project fields suggested: Component docs · Priority P3 · Effort S · Status Needs Review
Confidence: 0.90 (mixed)

Reasoning

component from path globs (docs); effort from diff stats (38+0 LOC, 1 files); LLM: Adds accumulator test coverage documentation to TEST_COVERAGE.md, a small documentation-only change.

If a label is wrong, remove it manually and ping @patty-chow so the rules can be tuned. The bot will not re-label items that already have component labels.

[eerxuan]

nit: add space before

[eerxuan]

• Add a rule to TEST_COVERAGE.md for deprecated features that we don't want to test. Add Code data type and mapReduce, reIndex, function, $accumulator, $where, currentOp command, filemd5, collStats command.
• Add a rule to list cross-cutting features:

### N. Foundational Spec Behaviors — Test Once

  **Rule**: Foundational spec behaviors (BSON type ordering, collation rules, GeoJSON parsing, etc.) are tested comprehensively in their dedicated directory and assumed consistent
  elsewhere. Consumers test only that they correctly delegate to the foundational behavior, not the foundational behavior itself.

  **Rationale**: We are compatibility tests, not comprehensive functional tests. Re-verifying foundational behavior across every consumer multiplies cases without adding signal — if the foundational behavior diverges, it shows
  in the foundational test, not in 50 downstream tests.

  **Examples**:
  - BSON type ordering → `tests/core/bson_types/`. Operators that use it (e.g. `$max`, `$gt`, `$sort`) get 1-2 wiring cases, not the full type-pair matrix.
  - Collation comparison → `tests/core/collation/`. Commands that accept `collation` test syntactic acceptance only. Sub-fields testing and semantic behavior is in 'tests/core/collation/'.
  - GeoJSON parsing and validation → `geospatial/specifiers/geometry/`. Geo operators that accept GeoJSON — test that the operator wires to the GeoJSON parser, not GeoJSON syntax tests.
  - Wire-protocol namespace validation → TBD. Commands that take a namespace as their first field — single representative case, not the full character matrix.
  - Field path validation  → Issue #118. 

  **Test naming convention**: wiring tests typically use the suffix `_bson_wiring.py` or `_<feature>_wiring.py`. Compare to `tests/core/operator/expressions/comparisons/gt/test_gt_bson_wiring.py` for the right
  shape — small, representative, explicitly named.

  **Carve-outs**: Per Rule 2's exception, parameters whose behavior genuinely varies per command (readConcern, writeConcern) are still tested exhaustively per command. The "test once" rule applies to behaviors that
   should be uniform across consumers.