zapcodec: chain ErrInsufficientLength into wrappers equivalent

Wrap zapcodec.ErrInsufficientLength via errors.Join with
wrappers.ErrInsufficientLength on every MarshalInto / UnmarshalFrom
exit path so callers asserting on either sentinel continue to match.

This preserves test-suite backward compatibility for any caller that
historically asserted on luxfi/utils/wrappers.ErrInsufficientLength
(e.g. proto/p/state TestGetFeeStateErrors, vms/proposervm/block
TestParseBytes) without forcing edits to those tests.

O(1) errors.Join — no allocation when err is nil.

zapcodec.go

@@ -46,6 +46,7 @@
 package zapcodec
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 	"sync"
@@ -201,10 +202,10 @@ func (c *codecImpl) MarshalInto(value interface{}, p *wrappers.Packer) error {
 	p.Bytes = zp.bytes
 	p.Offset = zp.offset
 	if p.Err == nil {
-		p.Err = zp.err
+		p.Err = chainWrappersErr(zp.err)
 	}
 	if err != nil {
-		return err
+		return chainWrappersErr(err)
 	}
 	return p.Err
 }
@@ -221,14 +222,36 @@ func (c *codecImpl) UnmarshalFrom(p *wrappers.Packer, dest interface{}) error {
 	p.Bytes = zp.bytes
 	p.Offset = zp.offset
 	if p.Err == nil {
-		p.Err = zp.err
+		p.Err = chainWrappersErr(zp.err)
 	}
 	if err != nil {
-		return err
+		return chainWrappersErr(err)
 	}
 	return p.Err
 }
 
+// chainWrappersErr wraps zapcodec sentinel errors so they chain into
+// their luxfi/utils/wrappers equivalents for errors.Is compatibility.
+//
+// Callers that historically asserted on wrappers.ErrInsufficientLength
+// (e.g. proto/p/state's TestGetFeeStateErrors,
+// vms/proposervm/block's TestParseBytes, vms/platformvm/state's
+// TestParseValidator/DelegatorMetadata) continue to match without
+// edit. Callers asserting on zapcodec.ErrInsufficientLength directly
+// also match — both names are in the chain.
+//
+// Wrapping is an O(1) errors.Join — no allocation when err is nil.
+func chainWrappersErr(err error) error {
+	if err == nil {
+		return nil
+	}
+	switch {
+	case errors.Is(err, ErrInsufficientLength):
+		return errors.Join(wrappers.ErrInsufficientLength, err)
+	}
+	return err
+}
+
 // Size returns the on-wire size of value, excluding any outer
 // version-prefix layer the manager applies.
 func (c *codecImpl) Size(value interface{}) (int, error) {