fix: use readable file stream instead of reading file into memory

[aavshr]

Note

Medium Risk
Changes the CLI upload path to use streams and toFile, which affects how file bodies and MIME types are constructed and could surface edge-case regressions in upload behavior (especially for multipart/large files). Scope is limited to the CLI uploader and keeps existing multipart/progress logic largely intact.

Overview
Switches CLI store uploads from readFile+in-memory File construction to streaming file reads via createReadStream and @mixedbread/sdk/uploads toFile, avoiding the Node Buffer 2 GiB limit and reducing memory usage for large files.

Refactors MIME handling from fixMimeTypes(File) to resolveMimeType(filePath) and updates both single-file (uploadFile) and batch uploads to compute sizes from stat().size before creating the stream-backed File used in client.stores.files.upload.

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