feat(providers/bedrock): prefix user content with sender name

Bedrock's Converse API multiplexes Claude, Llama, Titan, Mistral and
others through a single content-block protocol. None of those families
expose a per-message author identity in a unified way, so prefixing in
the text content block keeps multi-user attribution consistent
regardless of which underlying model the request lands on.

buildUserContent now passes msg through messageutil.ApplyUserNamePrefix
before constructing the text ContentBlockMemberText. Tool result paths
go through a separate code path (toolResultBlock) and are unaffected.
Image media handling is unchanged.

Tests:
- buildUserContent with Name set produces the prefixed text block
- buildUserContent without Name preserves byte-identical behavior
- existing image and base64 cases continue to pass

The Bedrock build is gated behind //go:build bedrock so this path is
only exercised in builds that opt in via that tag.

Refs #2702.

pkg/providers/bedrock/provider_bedrock.go

@@ -27,6 +27,7 @@ import (
 	"github.com/aws/aws-sdk-go-v2/service/bedrockruntime/types"
 
 	"github.com/sipeed/picoclaw/pkg/providers/common"
+	"github.com/sipeed/picoclaw/pkg/providers/messageutil"
 	"github.com/sipeed/picoclaw/pkg/providers/protocoltypes"
 )
 
@@ -319,13 +320,19 @@ func convertMessages(messages []Message) ([]types.Message, []types.SystemContent
 }
 
 // buildUserContent builds Bedrock content blocks for a user message.
+//
+// Sender attribution from msg.Name is rendered as a `[name] ` prefix on
+// the text block. Bedrock's Converse API has no per-message author identity
+// shared across all model families, so prefixing keeps the multi-user
+// attribution behavior consistent with the Anthropic adapter regardless
+// of which underlying model the request lands on.
 func buildUserContent(msg Message) []types.ContentBlock {
 	var content []types.ContentBlock
 
 	// Add text content
-	if msg.Content != "" {
+	if textValue := messageutil.ApplyUserNamePrefix(msg); textValue != "" {
 		content = append(content, &types.ContentBlockMemberText{
-			Value: msg.Content,
+			Value: textValue,
 		})
 	}
 

pkg/providers/bedrock/provider_bedrock_test.go

@@ -294,6 +294,42 @@ func TestBuildUserContent_SkipsNonBase64Data(t *testing.T) {
 	assert.Len(t, content, 1)
 }
 
+// TestBuildUserContent_UserNamePrefixed verifies that sender attribution
+// from Message.Name is rendered as a `[name] ` prefix on the Bedrock text
+// block. Bedrock's Converse API has no per-message author identity shared
+// across model families, so prefixing keeps multi-user disambiguation
+// consistent regardless of the underlying model (Claude, Llama, etc.).
+func TestBuildUserContent_UserNamePrefixed(t *testing.T) {
+	msg := Message{
+		Role:    "user",
+		Content: "My name is Alice",
+		Name:    "U_alice",
+	}
+
+	content := buildUserContent(msg)
+
+	assert.Len(t, content, 1)
+	textBlock, ok := content[0].(*types.ContentBlockMemberText)
+	require.True(t, ok)
+	assert.Equal(t, "[U_alice] My name is Alice", textBlock.Value)
+}
+
+// TestBuildUserContent_NoNameNoPrefix protects backward compatibility for
+// direct (single-user) channels: no sender, no prefix.
+func TestBuildUserContent_NoNameNoPrefix(t *testing.T) {
+	msg := Message{
+		Role:    "user",
+		Content: "Hello",
+	}
+
+	content := buildUserContent(msg)
+
+	assert.Len(t, content, 1)
+	textBlock, ok := content[0].(*types.ContentBlockMemberText)
+	require.True(t, ok)
+	assert.Equal(t, "Hello", textBlock.Value)
+}
+
 func TestBuildAssistantContent_SkipsEmptyToolName(t *testing.T) {
 	msg := Message{
 		Content: "Response",