test: add coverage for matching_markets method in Client

[Copilot]

Addresses missing test coverage for the matching_markets method added in the parent PR.

Changes

• Added test case in test/domeapi/client_test.rb verifying client.matching_markets returns Rubyists::Domeapi::MatchingMarkets instance
• Follows existing pattern from polymarket test for consistency

it 'returns a matching markets client' do
  _(client.matching_markets).must_be_instance_of Rubyists::Domeapi::MatchingMarkets
end

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