Fix #2051: [Feature]: LocalAgentRunner Agent Loop 应增加上下文保护与截断机制

[JiwaniZakir]

Closes #2051

概述 / Overview

请在此部分填写你实现/解决/优化的内容:
Summary of what you implemented/solved/optimized:

为 LocalAgentRunner（src/langbot/pkg/provider/runners/localagent.py）的 Agent 循环增加了两项上下文保护机制，防止因无限工具调用或超大工具返回结果导致的 token 失控与上下文溢出。

Two context-protection mechanisms have been added to the LocalAgentRunner agent loop to prevent unbounded token consumption and context window overflow:

1. 最大迭代次数限制 / Max iteration cap — 新增模块级常量 MAX_TOOL_ITERATIONS = 16，将 while pending_tool_calls: 改为 while pending_tool_calls and iteration_count < MAX_TOOL_ITERATIONS:，并在每轮递增计数器。LLM 持续返回 tool_calls（模型幻觉或工具重试）时循环不再无限运行。

2. 工具返回结果截断 / Tool result truncation — 新增模块级常量 MAX_TOOL_RESULT_LENGTH = 8000。在将工具结果序列化为 JSON 后，若字符数超出上限，截断并附加 \n...[truncated] 标记，防止单条工具消息独自撑满上下文窗口。

同时在 tests/unit_tests/provider/test_localagent.py 中新增 5 个单元测试，独立验证上述两段逻辑，不依赖完整的 langbot 导入链。

更改前后对比截图 / Screenshots

修改前 / Before:

while pending_tool_calls: — 无迭代上限；工具结果原样写入消息，无长度限制。

修改后 / After:

while pending_tool_calls and iteration_count < MAX_TOOL_ITERATIONS: — 最多执行 16 轮；工具结果超过 8000 字符时自动截断并追加 ...[truncated]。单元测试全部通过（见下方检查清单）。

检查清单 / Checklist

PR 作者完成 / For PR author

请在方括号间写x以打勾 / Please tick the box with x

• 阅读仓库贡献指引了吗？ / Have you read the contribution guide?
• 与项目所有者沟通过了吗？ / Have you communicated with the project maintainer?
• 我确定已自行测试所作的更改，确保功能符合预期。 / I have tested the changes and ensured they work as expected.

项目维护者完成 / For project maintainer

• 相关 issues 链接了吗？ / Have you linked the related issues?
• 配置项写好了吗？迁移写好了吗？生效了吗？ / Have you written the configuration items? Have you written the migration? Has it taken effect?
• 依赖加到 pyproject.toml 和 core/bootutils/deps.py 了吗 / Have you added the dependencies to pyproject.toml and core/bootutils/deps.py?
• 文档编写了吗？ / Have you written the documentation?

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This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/langbot/pkg/provider/runners/localagent.py	28.57%	5 Missing ⚠️

📢 Thoughts on this report? Let us know!

[RockChinQ]

麻烦贴一下修改前后截图证明功能正常，以便我们快速review并合入

[huanghuoguoguo]

这俩单元测试没有什么意义吧，而且变量定义在单元测试中

[huanghuoguoguo]

这个单元测试可以不需要。没有什么用。

[huanghuoguoguo]

#2057 同时参考这个pr内提出的问题。