fix(ga): retry metadata-only responses

[Fwind43]

Summary

Treat responses containing only <thinking> / <summary> metadata as incomplete instead of accepting them as final user-visible output.

Why

A real run ended after the model returned only:

<summary>转入真机更新，先核验SOP与构建态</summary>

There was no tool call and no user-visible answer. do_no_tool accepted it as a normal final result, leaving GA Admin with an empty terminal turn rendered as “正在等待该轮输出…”.

The metadata-only path now reuses the existing incomplete-response retry and exit policy.

Testing

• Replayed the exact production summary-only response and verified that it schedules a retry
• Exercised summary-only, case-insensitive summary tags, thinking-tag-only, separate response.thinking-only, and mixed metadata responses
• Verified summary plus visible text and ordinary final answers still finish normally
• Compiled all 72 Python files with py_compile
• git diff --check

Review principles

• Localized to metadata-only response handling in ga.py
• Reuses the existing retry boundary and exit policy
• No dependencies or unrelated changes