pxf: reject null on a (pxf.required) or @required field (full and lite) #189

Rule. A field with (pxf.required) or @required holds a value. A decoder rejects it when it is absent and also when it is bound to null, whatever the field's type and presence. Decided by the owner on 2026-11-04, reversing protowire#339 option 3 on this one point. Spec: trendvidia/protowire#569. Reference: trendvidia/protowire-go#268.

Measured on main (c685ff0) with the spec's nulls fixtures plus controls:

• Full tier (check-decode --format pxf-pb): r = null on optional int32 r [(pxf.required) = true] is accepted, and so is w = null on a required Int32Value. Both must be rejected. The controls behave (absent r rejected, r = 0 accepted).
• Lite tier (RequiredPxfCodec.unmarshal, throwaway test in lite-fixtures): r = null is accepted. The lite fixtures have no RequiredWrapper, so the wrapper case was not run.
• The generated NULLABLE_FIELDS table also changes: protowire#569 makes protoc-gen-pxf-java-meta leave required fields out of it.

Done when

• Both tiers reject null on a required field in a body, a nested body and a @dataset cell, naming the field (the spec asserts null is not allowed on required field "<name>").
• The vendored corpus picks up protowire#569's fixtures once it merges.