fix(udf): replace state during JSON decoding

[fallintoplace]

Summary

Make UDF JSON decoding replace receiver state instead of merging into it.

Problem

Several UDF UnmarshalJSON implementations decode through aliases of their existing receivers. Go leaves struct fields unchanged when they are absent from a new JSON document, so reusing a receiver can retain values from an earlier decode.

At the metadata level, a malformed document without definitions can pass validation using retained definitions, and the lazy definition index can remain stale. Exported nested values such as Parameter, DefinitionVersion, Definition, and DefinitionLogEntry also retain omitted optional fields when decoded directly.

Fix

Decode each value into fresh state and assign it to the receiver only after decoding succeeds. Top-level metadata is also validated before assignment, then initializes its definition index from the replacement state.

Tests

Added receiver-reuse coverage for top-level metadata and exported nested values. The tests verify omitted optional fields are cleared, cached indexes are replaced, and failed metadata decoding does not partially overwrite the last valid state.

go test ./udf -count=1