* fix: don't clobber native tool-call responses in after-LLM hooks
Registering any `after_llm_call` hook broke native tool execution: the
executor invokes `_setup_after_llm_call_hooks` on the intermediate
response that carries the model's tool calls, the non-str payload was
stringified for the response-rewrite pass, and the executor then treated
that string as a final answer instead of executing the tools. Structured
payloads (neither `str` nor `BaseModel`) now pass through untouched,
mirroring the isinstance guard `_invoke_after_llm_call_hooks` already
applies on the direct-call path; hooks still fire on the follow-up
textual response.
Fixes#6529
* style: shorten the tool-call guard comment
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