…errors (openclaw#14402)

* fix(feishu): chunk large documents for write/append to avoid API 400 errors

The Feishu API limits documentBlockChildren.create to 50 blocks per
request and document.convert has content size limits for large markdown.

Previously, writeDoc and appendDoc would send the entire content in a
single API call, causing HTTP 400 errors for long documents.

This commit adds:
- splitMarkdownByHeadings(): splits markdown at # or ## headings
- chunkedConvertMarkdown(): converts each chunk independently
- chunkedInsertBlocks(): batches blocks into groups of ≤50

Both writeDoc and appendDoc now use the chunked helpers while
preserving backward compatibility for small documents. Image
processing correctly receives all inserted blocks across batches.

* fix(feishu): skip heading detection inside fenced code blocks

Addresses review feedback: splitMarkdownByHeadings() now tracks
fenced code blocks (``` or ~~~) and skips heading-based splitting
when inside one, preventing corruption of code block content.

* Feishu/Docx: add convert fallback chunking + tests

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Co-authored-by: lml2468 <[email protected]>
Co-authored-by: Tak Hoffman <[email protected]>
(cherry picked from commit 4dc55ea)