Revert "fix: anthropic streaming content_block_start struct bug (#604)" (#609)

This reverts commit 449859f.

Author: zijiren233

core/relay/model/claude.go

@@ -54,8 +54,8 @@ type ClaudeImageSource struct {
 
 type ClaudeContent struct {
 	Type         string              `json:"type"`
-	Text         string              `json:"text"`
-	Thinking     string              `json:"thinking"`
+	Text         string              `json:"text,omitempty"`
+	Thinking     string              `json:"thinking,omitempty"`
 	Source       *ClaudeImageSource  `json:"source,omitempty"`
 	ID           string              `json:"id,omitempty"`
 	Name         string              `json:"name,omitempty"`