Check expect() inputs

[hadley]

Fixes #2247

[claude]

Claude finished @hadley's task —— View job

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Review Complete

• Read and analyze changed files
• Review implementation for potential bugs
• Check test coverage and snapshots
• Provide feedback

Issues Found

R/expectation.R:24 - check_character(failure_message) validates scalar character but should allow character vectors. The parameter documentation says failure_message is a "Message" (singular), but check_character() accepts any character vector. This inconsistency could be confusing.

The validation functions from the standalone types-check library are being used correctly and the test coverage is adequate.

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