Add MongoDB parity tests for $in/$nin regex members and empty $elemMatch

[pubkey]

Adds test/operators/query/mongodb-parity.test.ts: 18 test cases where every expectation encodes the behavior of a real mongod (verified against MongoDB 7.x via mongodb-memory-server, run side by side with mingo).

1. Regex values inside $in must pattern-match strings (MongoDB $in docs). mingo compares them literally instead:
   • {f: {$in: [/^ab/]}} on {f: 'abc'} → mongod true, mingo false
   • same for mixed lists ([/^a/, 5]) and string elements of array fields
2. $nin is the exact negation of $in, so it must exclude pattern-matching strings:
   • {f: {$nin: [/^ab/]}} on {f: 'abc'} → mongod false, mingo true
3. {$elemMatch: {}} matches arrays containing at least one document (object) element:
   • {f: {$elemMatch: {}}} on {f: [{x: 1}]} → mongod true, mingo false
   • scalar-only arrays correctly do not match in either engine

The remaining 12 tests are regression guards for behavior that already agrees with MongoDB and must keep passing after any fix: $eq with a regex operand stays a literal comparison, shorthand regex criteria keep pattern-matching, per-element array matching, type bracketing, and missing-field semantics for $ne/$eq: null.

Suggested fix direction (not included here): extend the shared $in/$nin membership helper so a RegExp list member also tests string values (and string elements of arrays) as a pattern, keeping the existing literal-equality path; make the empty-$elemMatch case match arrays containing a plain-object element.

Question: Shell we add something to the github actions CI that runs a real mongodb server to ensure exact equal bevahior between mongodb and mingo?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (10d07f1) to head (f57d258).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
+ Coverage   98.76%   98.79%   +0.02%     
==========================================
  Files         267      267              
  Lines        3651     3658       +7     
  Branches     1011     1014       +3     
==========================================
+ Hits         3606     3614       +8     
  Misses          2        2              
+ Partials       43       42       -1     

Flag	Coverage Δ
unittests	98.79% <100.00%> (+0.02%)	⬆️

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