Modernize RSpec tests to use expect syntax exclusively

[hownowstephen]

Summary

• Converted all .should / .should_not calls to expect().to / expect().not_to across all spec files
• Restricted spec_helper.rb to :expect syntax only (removed :should)
• No test logic changes — pure syntax modernization

Test plan

• All 130 examples pass
• Zero .should occurrences remain in spec files
• RuboCop clean

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Note

Low Risk
Low risk: changes are limited to test-suite assertion syntax and RSpec configuration, with no production code or API behavior modified.

Overview
Updates specs to use expect(...).to/not_to exclusively, replacing all legacy .should/.should_not assertions across api_client_spec, base_client_spec, and client_spec.

Tightens spec_helper.rb to enforce expect-only syntax for both expectations and mocks, preventing future should-style assertions from being added.

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