Preserve IOException cause in JWTParser decode errors

[asim-alam]

Changes

JWTParser.parsePayload and JWTParser.parseHeader catch the Jackson IOException
thrown while parsing a token's JSON, but rethrow it as a JWTDecodeException without
passing the original exception as the cause. As a result, Jackson's parsing details,
including the message and source location, are lost from the exception chain.

This PR forwards the caught IOException as the cause:

• Added a private decodeException(String json, Throwable cause) overload in JWTParser
  that builds the same message and passes the cause through.
• Updated the two catch (IOException e) sites in parsePayload and parseHeader to use it.
• No public API change: JWTDecodeException already has a (String, Throwable) constructor.
• No behavior change beyond the fix: the exception type and message are unchanged;
  only getCause() on the parse-failure path changes from null to the underlying Jackson exception.

Before:

getCause() : null

After:

getCause() : com.fasterxml.jackson.core.JsonParseException: Unexpected close marker '}' ...

References

Found via static analysis: PMD's PreserveStackTrace rule flags both catch sites.

No related support ticket or community post.

Testing

• Added JWTParserTest.shouldPreserveCauseWhenParsingInvalidJson, which asserts the thrown
  JWTDecodeException's cause is the underlying IOException.

• Ran the full module suite with ./gradlew :java-jwt:test: 680 tests, 0 failures

• This change adds unit test coverage

• This change has been tested on the latest version of the platform/language

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors