Fixes #601: Jackson subtype schema unions are non-deterministic

[beseder1]

(fixes: #601)

With the current implementation of the RecordVisitor, Avro schema unions are handled in a non-deterministic way. This can cause deserialization errors when different AvroMapper instances are used for serialization and deserialization.

Specifically, because the subtype ordering is unstable, deserialization may attempt to resolve a serialized object against the wrong subtype, resulting in errors.

This PR makes the ordering deterministic by using an ArrayList instead of a Set, ensuring consistent behavior across serialization and deserialization.

[cowtowncoder]

Good start! PR much appreciated.
I think we can avoid separate de-duping by checking for existence on-the-fly, but aside from that I think this makes sense.

One process thing: we need CLA (from either https://github.com/FasterXML/jackson/blob/main/contributor-agreement.pdf (individual) or https://github.com/FasterXML/jackson/blob/main/contributor-agreement.pdf (Corporate CLA)). This is only needed once per contributor, one is good for all future contributions.

The usual way is to print, fill & sign, email to cla at fasterxml dot com. Once this is done I can proceed with merging (pending final review etc).

Looking forward to merging this PR (and backporting in 2.19, I think).

[cowtowncoder]

LGTM

[cowtowncoder]

Ok ready to merge once we get CLA!

Thank you again @beseder1 .