feat(drive): support multipart upload for files larger than 20MB

[xoyowade]

Summary

• drive +upload previously rejected files exceeding 20MB with a validation error
• Now files > 20MB automatically use the three-step chunked upload API (upload_prepare → upload_part × N → upload_finish), removing the size ceiling for Drive uploads
• Small files (≤ 20MB) continue to use the single-request upload_all endpoint unchanged

Test plan

• Unit test TestDriveUploadLargeFileUsesMultipart validates the full 3-step multipart flow with mocked API responses
• All existing drive tests continue to pass
• Real upload tested: 189MB file → 48 blocks × 4MB, successfully uploaded to a live Feishu tenant (file_token: LVb3bP6Nmo6EoKxzwgbcPz4xnCe)
• Small file (5MB) verified to still use single upload_all path

🤖 Generated with Claude Code

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Codecov Report

❌ Patch coverage is 60.56338% with 28 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@d2ad5e4). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
shortcuts/drive/drive_upload.go	60.56%	18 Missing and 10 partials ⚠️
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #43   +/-   ##
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  Coverage        ?   47.94%           
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  Files           ?      280           
  Lines           ?    23045           
  Branches        ?        0           
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  Hits            ?    11048           
  Misses          ?    11004           
  Partials        ?      993           

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