Add cross-cutting test coverage: system limits and collection name validation

[eerxuan]

Summary

Two areas of cross-cutting test coverage that don't belong in per-operator or per-command tests.

1. Collection name validation

Shared tests for collection name handling across all collection commands (create, drop, rename, etc.):

• Unicode names
• Dots in names
• system. prefix behavior
• $ prefix handling
• Empty string
• Null bytes
• Max length names

Location: tests/core/collections/test_collection_name_validation.py

These are feature spec behaviors — how the system handles special characters in collection names.

2. System limits

Tests for documented MongoDB limits that cut across features:

• Max document size (16MB)
• Max namespace length
• Max number of indexes per collection
• Max pipeline stage count
• Max nesting depth

Location: tests/system/limits/

Reference: https://www.mongodb.com/docs/manual/reference/limits/

Notes

• Collection name validation is shared behavior — test once, not per-command
• System limits are compatibility-relevant (engines should enforce the same limits)
• Both are distinct from per-operator edge cases already covered in TEST_COVERAGE.md