test: improve coverage from 53% to 94%

[basgys]

Summary

• Restructure TestIs and TestAs to use t.Run with descriptive scenario names instead of index-based error messages
• Add TestWith to verify all With* functions preserve the parent cause in the error chain
• Add TestErrorMessage to validate the human-readable output of every error type (no violations, with violations, wrapping a cause)
• Add TestViolationString to cover FieldViolation, QuotaViolation, PreconditionViolation, and ConflictViolation string formatting
• Add missing coverage for IsUnimplemented and AsUnimplemented

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Note

Low Risk
Low risk: changes are confined to faults_test.go and only increase assertions around error classification, wrapping/unwrapping behavior, and message formatting.

Overview
Significantly expands and restructures faults_test.go to use named t.Run scenarios and to cover more cases for Is*/As* helpers, including nested fmt.Errorf wrapping, negative cases, and the previously untested Unimplemented helpers.

Adds new tests verifying With* wrappers preserve the original cause via errors.Is, validates the exact Error() strings for singleton and constructed faults (including violations and wrapped causes), and covers String() formatting for all violation types.

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