test: add coverage for matching_markets client method

[Copilot]

Adds missing test coverage for the matching_markets method in the Client class, as identified in PR #7 review feedback.

Changes

• Added test case in test/domeapi/client_test.rb verifying client.matching_markets returns a Rubyists::Domeapi::MatchingMarkets instance
• Mirrors existing test pattern for client.polymarket on line 32-34

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

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