docs: fix Javadoc warnings in field validation and deserialization limits

[ar-agt]

Summary

Resolve all eight Javadoc warnings reproduced on main (57834413c).

• Document the parameters and ISOException conditions of ISOFieldPackager.checkAvailable and checkUnpackedLength.
• Document the DeserializationLimits compact constructor, its parameters, and rejection of negative limits.
• Documentation only; no runtime behavior or build warning suppression changes.

Verification

• Before: ./gradlew javadoc --console=plain succeeded with eight Javadoc warnings.
• After: ./gradlew javadoc aggregatedJavadoc --console=plain succeeded with zero Javadoc warnings.
• git diff --check passed.
• Used Java 26.0.2-amzn and Gradle 9.7.1 from .sdkmanrc.
• Unit tests not rerun for this documentation-only change. Java compilation still prints its existing deprecation/unchecked-operation notes; these are not Javadoc warnings.