Fix union branch resolution for primitive promotion

[arkorwan]

Summary

This change fixes fastserde union resolution for schema evolution cases where a writer primitive should be promoted to a compatible reader primitive inside a union.

Previously, fastserde could fail on cases like:

writer: ["null", "int"]
reader: ["null", "long"]

What changed

• SchemaAssistant.areTypesCompatible(...) now recognizes valid primitive promotions in addition to exact matches.
• Reader union branch selection now uses that compatibility logic consistently.
• Exact matches still win over promotions in case there are multiple matches.
• Added a regression test for ["null","int"] -> ["null","long"].