Allow zero chunk overlap in recursive chunker

[Soulter]

fixes: #3207

Motivation

• Fix issue where chunk_overlap=0 was treated as falsy and fell back to default overlap, preventing users from setting zero overlap.
• The bug appeared in the recursive character chunker when using or to select fallback values.

Description

• Replace chunk_size = chunk_size or self.chunk_size and overlap = overlap or self.chunk_overlap with explicit if ... is None checks in astrbot/core/knowledge_base/chunking/recursive.py.
• This change ensures that chunk_overlap=0 is accepted as a valid value while only None triggers the default.
• The only modified file is astrbot/core/knowledge_base/chunking/recursive.py and the change is a small defensive check.

Testing

• No automated tests were executed as part of this rollout.
• The change is limited to value selection logic and does not add new behavior beyond allowing zero overlap.

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Codex Task

Summary by Sourcery

Bug Fixes:

• 修复了对 chunk_size 和 chunk_overlap 的处理方式，使得显式传入 0 时不再触发使用默认值。

Original summary in English

Summary by Sourcery

Bug Fixes:

• Fix handling of chunk_size and chunk_overlap so that explicitly passing 0 no longer triggers use of the default values.

[sourcery-ai]

你好——我在这里给出了一些总体性的反馈：

• 建议对一些无效组合添加防御性检查，比如 chunk_size <= 0 或 overlap >= chunk_size，这样可以避免当用户传入边界值时，这个循环出现步长为非正数或其他意料之外的行为。

给 AI Agent 的提示词

Please address the comments from this code review:

## Overall Comments
- Consider adding defensive checks for invalid combinations like `chunk_size <= 0` or `overlap >= chunk_size` so this loop cannot end up with a non-positive step or unexpected behavior when users pass edge-case values.

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Original comment in English

Hey - I've left some high level feedback:

• Consider adding defensive checks for invalid combinations like chunk_size <= 0 or overlap >= chunk_size so this loop cannot end up with a non-positive step or unexpected behavior when users pass edge-case values.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider adding defensive checks for invalid combinations like `chunk_size <= 0` or `overlap >= chunk_size` so this loop cannot end up with a non-positive step or unexpected behavior when users pass edge-case values.

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