Java nit: replace the fragile SerializerConfigImpl cast for Kryo serializer registration

[atomicdragonranch]

From a 2026-07-03 code review. Low-priority polish:

• StreamEventDeserializer tracks deserializationErrors as a plain transient long (invisible in the Flink UI) and uses a % 100 magic number , use a Flink Counter.
• ProtobufKryoSerializer.read() calls type.getMethod("parseFrom", ...) reflectively per record , cache the Method.
• The 30s window is a hardcoded literal while every other tunable is externalized.
• ProtobufKryoSerializer (the most bug-prone class) has NO round-trip test , add one.
• ((SerializerConfigImpl) ...) cast reaches into an Impl class , fragile across Flink versions.

[atomicdragonranch]

Deserialization-error Counter shipped in PR #74 (now a Flink metric, with a test). Remaining in #66: the reflective parseFrom cache (done in #72), the externalized window (done in #72), the round-trip serializer test (done in #72). Only the SerializerConfigImpl cast is left , keeping this open just for that. It needs investigating whether Flink 2.0 exposes a public Kryo-serializer registration path (likely a move to config-based pipeline.serialization-config).