Add ActiveHash::Relation#or to support OR-like queries

[opsys-saito]

Summary

This PR adds support for ActiveHash::Relation#or, allowing OR-like queries similar to ActiveRecord:

Model.where(condition_a).or(Model.where(condition_b))

The implementation returns a new relation representing the union of both result sets, deduplicated by id.

Behavior

• Supports chaining:

  Model.where(a: 1).or(Model.where(a: 2)).where(b: 3)

• Deduplicates records based on id
• Raises an ArgumentError when combining relations from different models

Implementation Notes

Since ActiveHash operates entirely in memory, this implementation performs a union of the evaluated result sets rather than merging condition trees.

The returned value remains an ActiveHash::Relation, preserving chainability and compatibility with existing APIs.

Tests

Added specs covering:

• Union of two relations
• Deduplication behavior
• Chainability after or
• Error handling for mismatched models