fix: deduplicate assistant content in multi-tool turns to prevent premature loop exit

- process.go: only emit resp.Content on the first tool call per batch,
  subsequent assistant messages use empty content (serialized as null)
- provider.go: MarshalJSON outputs null content when empty with tool_calls
  to comply with DeepSeek/OpenAI expected format
- output.go: update parameter interface (payload/type/meta) to match
  tool definitions (uncommitted from previous refactor)

internal/agent/api/provider.go

@@ -47,8 +47,10 @@ func (m Message) MarshalJSON() ([]byte, error) {
 	}
 	if len(m.Blocks) > 0 {
 		raw["content"] = m.Blocks
-	} else {
+	} else if m.Content != "" || len(m.ToolCalls) == 0 {
 		raw["content"] = m.Content
+	} else {
+		raw["content"] = nil
 	}
 	if m.ReasoningContent != "" {
 		raw["reasoning_content"] = m.ReasoningContent