client: Honor IsError and extract JSON from tool text

Fold Jerry dump into toolJSON: TrimSpace IsError text, concat JSON
via rawJSON, and unit tests mirroring IsError=true /
StructuredContent=null / Text=`end must be after start` / GetError=nil.

client/session.go

@@ -100,18 +100,18 @@ func toolJSON(res *mcp.CallToolResult) (json.RawMessage, error) {
 	if res == nil {
 		return nil, fmt.Errorf("empty tool result")
 	}
-	// GetError is only set by SetError on the server; the err field is not marshaled to clients.
+	// GetError is set by SetError on the server; the err field is not marshaled to clients.
 	if err := res.GetError(); err != nil {
 		return nil, err
 	}
 	texts, types := collectTextAndTypes(res)
 	concat := strings.Join(texts, "")
-	// IsError is the client-visible soft-fail flag; error text lives in Content.
 	if res.IsError {
-		if concat == "" {
-			return nil, fmt.Errorf("tool error")
+		msg := strings.TrimSpace(concat)
+		if msg == "" {
+			msg = "tool error"
 		}
-		return nil, fmt.Errorf("%s", concat)
+		return nil, fmt.Errorf("%s", msg)
 	}
 	if res.StructuredContent != nil {
 		b, err := json.Marshal(res.StructuredContent)
@@ -121,16 +121,16 @@ func toolJSON(res *mcp.CallToolResult) (json.RawMessage, error) {
 		return json.RawMessage(b), nil
 	}
 	for _, t := range texts {
-		if json.Valid([]byte(t)) {
-			return json.RawMessage(t), nil
+		if raw, ok := rawJSON(t); ok {
+			return raw, nil
 		}
 	}
+	if raw, ok := rawJSON(concat); ok {
+		return raw, nil
+	}
 	if concat == "" {
 		return json.RawMessage(`{}`), nil
 	}
-	if json.Valid([]byte(concat)) {
-		return json.RawMessage(concat), nil
-	}
 	if raw := extractFirstJSON(concat); raw != nil {
 		return raw, nil
 	}
@@ -138,8 +138,16 @@ func toolJSON(res *mcp.CallToolResult) (json.RawMessage, error) {
 	if res.StructuredContent != nil {
 		structured = "present"
 	}
-	return nil, fmt.Errorf("non-json tool result (isError=%v types=[%s] structured=%s textLen=%d prefix=%q)",
-		res.IsError, strings.Join(types, " "), structured, utf8.RuneCountInString(concat), runePrefix(concat, 80))
+	return nil, fmt.Errorf("non-json tool result (isError=%v types=%v structured=%s textLen=%d prefix=%q)",
+		res.IsError, types, structured, utf8.RuneCountInString(concat), runePrefix(concat, 80))
+}
+
+func rawJSON(s string) (json.RawMessage, bool) {
+	b := []byte(s)
+	if !json.Valid(b) {
+		return nil, false
+	}
+	return json.RawMessage(b), true
 }
 
 func collectTextAndTypes(res *mcp.CallToolResult) (texts, types []string) {