fix(localagent): stop re-seeding stream accumulator with previous rou…

[MIKAZE3]

…nd content

When the LLM (e.g. MiniMax-M3) returns multiple rounds after tool calls, the _StreamAccumulator was initialized with initial_content=first_content, causing every subsequent round to repeat the entire first message. Remove the re-seeding so each round starts with a clean accumulator.

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[dadachann]

这个拆分粒度没问题，但分支已与 master 冲突，且 localagent.py 最近有后续改动。请 rebase 后补一个多轮 tool call 内容不重复的回归测试，再继续 review。

[codecov]

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