exportJars := true and documentPart type="file" is broken

If exportJars is true and documentPart type="file" then we fail to get the length of the TDML file because it is inside a jar with the "jar" protocol. This leads to an assertion because the length ends up as -1. The code of interest is here:

https://github.com/apache/daffodil/blob/main/daffodil-tdml-lib/src/main/scala/org/apache/daffodil/tdml/TDMLRunner.scala#L2249-L2262

We should fix this so that we can get the length of TDML resources in jars and so that the TDML runner can use test files from a jar. Or alternatively, just allow length to be unknown in this case and tests have no limit. That is not as ideal, since it means different behavior depending on jar vs non-jar test files.