* fix(feishu): normalize media upload multipart data
Convert Feishu SDK multipart Buffer upload parts into explicit FormData
before they reach the wrapped HTTP transport. The SDK upload helpers
pass multipart data as a plain object with Buffer media parts; relying on
implicit serialization in the timeout/proxy-aware HTTP wrapper was fragile
and caused file/image delivery failures (400 volc-dcdn / write ECONNRESET)
even though the same credentials succeeded via a standalone SDK upload.
Adds normalizeMultipartUploadData, applied on the request path of the
shared Feishu HTTP instance, plus a regression test covering the
multipart-to-FormData normalization.
Rebased onto current origin/main, which replaced the old synchronous
injectTimeout path with the async proxy-aware injectRequestOptions flow;
the multipart normalization is now applied before that flow.
* fix(feishu): scope multipart media normalization
* test(feishu): mark multipart auth fixture synthetic
* test(feishu): use field-shaped secret fixtures
* fix(feishu): validate multipart upload endpoints
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Co-authored-by: Peter Steinberger <steipete@gmail.com>
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