Add support for specifying distinct columns in result queries

[Copilot]

Addresses the issue where users could only use distinct: true to generate SELECT DISTINCT on all columns, with no way to specify which specific columns should be included in the DISTINCT clause.

What's Changed

Previously, users were limited to:

@q.result(distinct: true)  # SELECT DISTINCT * FROM table
@q.result                  # No DISTINCT clause

Now users can specify exactly which columns to use in the DISTINCT clause:

# Single column as string or symbol
@q.result(distinct: 'name')
@q.result(distinct: :email)

# Multiple columns as array
@q.result(distinct: [:name, :email])
@q.result(distinct: ['name', 'email', 'created_at'])

# Explicit control over DISTINCT behavior
@q.result(distinct: false)  # No DISTINCT (explicit)
@q.result(distinct: nil)    # No DISTINCT
@q.result(distinct: [])     # SELECT DISTINCT * (fallback)

Implementation Details

• Core changes: Modified lib/ransack/adapters/active_record/context.rb to handle various distinct option types
• Backward compatible: All existing distinct: true usage continues to work unchanged
• Flexible input: Accepts strings, symbols, arrays, and mixed types
• Edge case handling: Properly handles false, nil, empty arrays
• ActiveRecord native: Uses ActiveRecord's built-in distinct(*columns) method

Benefits

This eliminates the need for workarounds that users previously had to employ:

# Before: Manual workarounds
@q.result(distinct: true)
  .select('people.*, articles.name, articles.description')

# Before: Using group instead of distinct  
@q.result.group('people.id').includes(:articles)

# Now: Direct and clean
@q.result(distinct: [:name, :email])

Testing

Added comprehensive test coverage including:

• All supported data types (string, symbol, array, boolean, nil)
• Edge cases and mixed type arrays
• SQL generation validation
• Backward compatibility verification

Documentation

Updated the "Problem with DISTINCT selects" section in the documentation with examples and a complete reference of supported option types.

Fixes the issue where users needed to work around distinct limitations with manual column selection or alternative approaches like grouping.

Fixes #1533

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