docs: add comprehensive documentation to ApiClient class

[Copilot]

Addresses feedback requesting documentation for the ApiClient base class and its automatic prefix extraction mechanism.

Changes

• Class-level documentation: Explains ApiClient provides automatic URL prefix configuration by extracting from module hierarchy
• Method documentation: Documents prefix, prefix=, and initialize with YARD annotations
• Examples: Shows prefix extraction (Rubyists::Kalshi::Search::Client → "search") and custom override pattern

# Base class for API client wrappers that automatically configure URL prefixes
#
# @example Using ApiClient with a Search namespace
#   # For Rubyists::Kalshi::Search::Client, the prefix will be "search"
#   class Search::Client < ApiClient
#     # API calls will automatically use /search/ prefix
#   end
#
# @example Overriding the automatic prefix
#   class CustomClient < ApiClient
#     self.prefix = 'custom_api'
#   end
class ApiClient
  # Automatically extract the URL prefix from the module hierarchy
  #
  # The prefix is derived by taking the second-to-last component of the
  # fully qualified class name and converting it to lowercase. For example:
  #   - Rubyists::Kalshi::Search::Client => "search"
  #   - Rubyists::Kalshi::Market::Client => "market"
  #
  # @return [String] the URL prefix for API calls
  def self.prefix
    @prefix ||= to_s.split('::')[-2].downcase
  end
  
  # ...
end

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[bougyman]

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[bougyman]

@copilot This PR rejected for not meeting the standard for commit messages.