fix: Add path prefix presence flag and tests for AppFileInfo

[anxinxu]

Description

This PR improves AppFileInfo handling for path_prefix_index presence semantics while preserving backward compatibility.

• Added hasPathPrefixIndex to AppFileInfo so callers can explicitly distinguish whether path_prefix_index is present in the protobuf payload.
• Kept pathPrefixIndex as Int to avoid breaking existing callers that rely on the previous API shape.
• Added focused unit tests to cover both key cases:
  • path_prefix_index is absent -> hasPathPrefixIndex == false
  • path_prefix_index is present and equals 0 -> hasPathPrefixIndex == true and pathPrefixIndex == 0

Checklist

• Code compiles correctly
• All tests passing
• Samples run successfully
• Extended the README / documentation, if necessary

[anxinxu]

@LossyDragon Could you please review this PR? It adds a backward-compatible presence flag for pathPrefixIndex in AppFileInfo and includes tests for both missing and zero-index cases. Thanks!

[LossyDragon]

Mind changing the test class to Java? I try to keep the tests in Java (for now) to ensure Java/Kotlin compatibility.

package in.dragonbra.javasteam.steam.handlers.steamcloud;

import in.dragonbra.javasteam.protobufs.steamclient.SteammessagesCloudSteamclient;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

public class AppFileInfoTest {

    @Test
    public void hasPathPrefixIndexIsFalseWhenFieldIsMissing() {
        var response = SteammessagesCloudSteamclient.CCloud_AppFileInfo.newBuilder()
                .setFileName("save.dat")
                .build();

        var info = new AppFileInfo(response);

        Assertions.assertFalse(info.getHasPathPrefixIndex());
    }

    @Test
    public void hasPathPrefixIndexIsTrueWhenIndexIsZero() {
        var response = SteammessagesCloudSteamclient.CCloud_AppFileInfo.newBuilder()
                .setFileName("save.dat")
                .setPathPrefixIndex(0)
                .build();

        var info = new AppFileInfo(response);

        Assertions.assertTrue(info.getHasPathPrefixIndex());
        Assertions.assertEquals(0, info.getPathPrefixIndex());
    }
}