fix: Align errorMessage param type in validate and requireNotNull as same as their suspend counterparts

[Vijay0710]

🎯 Goal

validate and requireNotNull accepted a lazy lambda crossinline errorMessage: () -> String
while their suspend counterparts suspendValidate and suspendRequireNotNull accepted a plain
String. This inconsistency made the API surface asymmetric and the existing KDocs actively
misleading — they still described errorMessage as a lambda even after the implementation
diverged. This PR aligns the non-suspend functions with the suspend ones and corrects the stale
documentation.

🛠 Implementation details

• Changed errorMessage in validate and requireNotNull from
  crossinline errorMessage: () -> String to errorMessage: String
• Removed the now-redundant callsInPlace(errorMessage, ...) contract clause from both functions
• Fixed @param errorMessage KDoc in validate: was "A lambda that provides the error message
  when validation fails" → now "The error message when validation fails."
• Fixed @param errorMessage KDoc in requireNotNull: was "A lambda that provides the error
  message when the selected value is null" → now "The error message when the selected value
  is null."
• Updated call sites in ResponseTransformerTest from trailing lambda syntax to plain String
• Regenerated .api dump files via ./gradlew apiDump

✍️ Explain examples

Before:

apiResponse.validate({ it.length > 2 }) { "Too short" }
apiResponse.requireNotNull({ it.field }) { "Value is null" }

After:

apiResponse.validate({ it.length > 2 }, errorMessage = "Too short")
apiResponse.requireNotNull({ it.field }, "Value is null")

// Suspend variants were already consistent — unchanged:
apiResponse.suspendValidate({ it.length > 2 }, "Too short")
apiResponse.suspendRequireNotNull({ it.field }, "Value is null")