fix(fanout): keep string value in fanout proto, skip binary content

Revert fanout proto value field to string since plugins expect text.
Guard the assignment with UTF-8 validation so binary inline content
is only available via the Content bytes field.

Signed-off-by: Miguel Martinez Trivino <miguel@chainloop.dev>

Author: migmartri

app/controlplane/plugins/sdk/v1/plugin/api/fanout.pb.go

@@ -109,7 +109,7 @@ message ExecuteRequest {
     bytes content = 1;
     string name = 2;
     string type = 3;
-    bytes value = 4;
+    string value = 4;
     string hash = 5;
     bool uploaded_to_cas = 6;
     string file_name = 7;

app/controlplane/plugins/sdk/v1/plugin/api/fanout.proto

@@ -1,5 +1,5 @@
 //
-// Copyright 2023 The Chainloop Authors.
+// Copyright 2023-2026 The Chainloop Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@ package api
 
 import (
 	"errors"
+	"unicode/utf8"
 
 	"github.com/chainloop-dev/chainloop/app/controlplane/plugins/sdk/v1"
 	"github.com/chainloop-dev/chainloop/pkg/attestation/renderer/chainloop"
@@ -148,11 +149,18 @@ func MetadataProtoToSDK(in *ExecuteRequest_Metadata) *sdk.ChainloopMetadata {
 }
 
 func MaterialSDKToProto(in *sdk.ExecuteMaterial) *ExecuteRequest_NormalizedMaterial {
+	// The fanout proto uses string for value since plugins expect text.
+	// Binary inline content is available in the Content bytes field instead.
+	var value string
+	if utf8.Valid(in.Value) {
+		value = string(in.Value)
+	}
+
 	return &ExecuteRequest_NormalizedMaterial{
 		Content:       in.Content,
 		Name:          in.Name,
 		Type:          in.Type,
-		Value:         in.Value,
+		Value:         value,
 		FileName:      in.Filename,
 		Hash:          in.Hash.String(),
 		UploadedToCas: in.UploadedToCAS,
@@ -171,7 +179,7 @@ func MaterialProtoToSDK(in *ExecuteRequest_NormalizedMaterial) *sdk.ExecuteMater
 		NormalizedMaterial: &chainloop.NormalizedMaterial{
 			Name:          in.Name,
 			Type:          in.Type,
-			Value:         in.Value,
+			Value:         []byte(in.Value),
 			Filename:      in.FileName,
 			UploadedToCAS: in.UploadedToCas,
 			Hash:          &hash,

app/controlplane/plugins/sdk/v1/plugin/api/translation.go

@@ -102,10 +102,6 @@ modules:
       except:
         - EXTENSION_NO_DELETE
         - FIELD_SAME_DEFAULT
-      ignore_only:
-        # string→bytes is wire-compatible; internal plugin SDK, not public API
-        FIELD_SAME_TYPE:
-          - app/controlplane/plugins/sdk/v1/plugin/api/fanout.proto
   - path: pkg/attestation/crafter/api
     lint:
       use: