Propagate Java stack traces from JVM upcalls into DataFusionError

[andygrove]

Follow-up from PR #46 review (thread).

Problem

When a Java-implemented upcall throws (currently: scalar UDFs via JniBridge, but the same will apply to any future upcall — table providers, aggregate UDFs, etc.), the native side surfaces only the exception class name and getMessage() via DataFusionError::Execution. The Java stack trace is discarded, which makes debugging much harder than necessary.

Proposal

Capture the throwable's stack trace on the JVM side and include it in the error string returned to native code. Make verbosity configurable per SessionContext so production callers can opt out — e.g. an enum like message | full | none, default full.

Scope

Should cover all JVM→native upcall paths, not just scalar UDFs. A shared helper in org.apache.datafusion.internal.JniBridge (or a new JniErrors utility) that converts a Throwable to the configured representation, used wherever upcalls catch exceptions.

References

• pgwhalen's session-configurable approach in his community binding: https://github.com/pgwhalen/datafusion-java/blob/1c20733aa8b008315af6092a912b58ef3df6a482/datafusion-ffi-java/src/main/java/org/apache/arrow/datafusion/config/ConfigOptions.java#L50-L54