xet: Commit operation to edit part of file (optimized for handling edits at beginning of file)

[coyotte508]

Fix #1705

// Edit a file by adding prefix & suffix
await commit({
  repo,
  accessToken: "hf_...",
  operations: [{
    type: "edit",
    originalContent: originalFile,
    edits: [{
      start: 0,
      end: 0,
      content: new Blob(["prefix"])
    }, {
      start: originalFile.length,
      end: originalFile.length,
      content: new Blob(["suffix"])
    }]
  }]
})

// Edit first kB of file
await commit({
  repo,
  accessToken: "hf_...",
  operations: [{
    type: "edit",
    originalContent: originalFile,
    edits: [{
      start: 0,
      end: 1000,
      content: new Blob(["blablabla"])
    }]
  }]
})

cc @mishig25 @assafvayner @jsulz

also

• fallback to LFS for non-xet repos (even if useXet is true)
• remove invalid Accepts header

How it works under the hood

• we load dedup info for first chunk of original file content if it's changed
• we upload the blob as normal

Todo

currently blob is being processed twice, once for sha256 and once for hashing. The file should be processed only once (maybe after #1704 - using workers for different processes)

[HuggingFaceDocBuilderDev]

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