Fix remaining should syntax in timestamp specs

[hownowstephen]

Summary

• Convert 4 remaining lambda { }.should raise_error calls to expect { }.to raise_error in timestamp validation specs
• These were missed in Modernize RSpec tests to use expect syntax exclusively #146 and fail on modern RSpec

Test plan

• CI passes all specs

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Note

Low Risk
Low risk because changes are confined to RSpec expectation syntax in specs and do not affect runtime code paths.

Overview
Updates timestamp validation specs in spec/client_spec.rb to use modern RSpec expect { ... }.to raise_error syntax instead of deprecated lambda { ... }.should raise_error, fixing compatibility with current RSpec versions.

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