feat: add flag for pending uploads

[jguo144]

Description

Set cloudstorage_multipart_pending whenever a multipart upload starts so downstream hooks know the file is not committed yet. Clear the flag with resource_patch after finish_multipart and abort_multipart so cancelled or finished uploads do not leave resources stranded.

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[nolanpruett]

This wasn't really in the scope of this change, but I think it may be worth taking a look and addressing:

clean_multipart (multipart.py:364) deletes expired uploads but never calls resource_patch to clear the pending flag. A resource whose multipart upload expires silently will be stuck with cloudstorage_multipart_pending="True" indefinitely — effectively blocking xloader forever. This needs the same flag-clearing logic added to clean_multipart.

[jguo144]

I’m not a fan of calling resource_patch in the clean_multipart API as it may operate across multiple resources and unnecessarily update their “last modified” timestamps. It may trigger side effects for something that’s really for cleanup.
Instead I added an ignore_pending argument in xloader, similar to the existing ignore_hash option.