Add $rand expression tests

[danielfrankcom]

This change adds tests for the $rand expression operator. These tests were originally written by @mitchell-elholm.

Test files added:

• Argument handling tests for empty array/object syntax, invalid argument types (all BSON types, field references, $missing), and non-empty array/object error codes
• Return value tests for type (double), range [0, 1), and per-invocation independence
• Combination tests covering arithmetic operators ($multiply, $add, $subtract, $pow, $mod, $divide), conditional operators ($cond), type conversions ($toString, $concat), $let variable binding and reuse, $switch, $filter, object expressions, and deep nesting
• Pipeline stage integration tests for $project, $group, and $match covering per-document independence, statistical distribution, uniform distribution, range validation, null field handling, and empty collection behavior — placed at operator level to match repo conventions

Ref: #17