Add llm message validate

internal/llm/llm.go

@@ -4,6 +4,7 @@ package llm
 
 import (
 	"encoding/json/v2"
+	"fmt"
 	"strings"
 )
 
@@ -524,6 +525,16 @@ func (m *Message) Text() string {
 	return b.String()
 }
 
+// Validate checks conformance to the OpenAI API specification.
+func (m *Message) Validate() error {
+	if len(m.ToolCalls) != 0 && m.Role != RoleAssistant {
+		return fmt.Errorf(
+			"%q message with tool calls", m.Role,
+		)
+	}
+	return nil
+}
+
 // UnmarshalJSON decodes a message while normalizing a string-valued content
 // field to one text ContentPart. An array-valued content field is decoded in
 // order. If reasoning_content is absent, reasoning is copied into