fix(providers): handle nil input in GLM series tool_use blocks

- add defensive nil check for tool call Arguments field
- replace nil input with empty object to comply with Anthropic spec
- prevent API errors when GLM models return null input in tool_use blocks

Zhipu AI's GLM series models may return tool_use blocks with null input field,
which causes their API to reject subsequent requests with error:
"ClaudeContentBlockToolResult object has no attribute id"

This fix ensures compatibility by converting nil inputs to empty objects {},
matching the Anthropic Messages API specification while maintaining backward
compatibility with other providers.

pkg/providers/anthropic_messages/provider.go

@@ -221,11 +221,17 @@ func buildRequestBody(
 
 			// Add tool_use blocks
 			for _, tc := range msg.ToolCalls {
+				// Handle nil Arguments (GLM-4 may return null input)
+				input := tc.Arguments
+				if input == nil {
+					input = map[string]any{}
+				}
+
 				toolUse := map[string]any{
 					"type":  "tool_use",
 					"id":    tc.ID,
 					"name":  tc.Name,
-					"input": tc.Arguments,
+					"input": input,
 				}
 				content = append(content, toolUse)
 			}