Force bulk upload temp files onto S3 when env still says local

[bernardhanna]

Summary

• Fix bulk upload: store temp files on S3 #3675 defaulted to S3, but Forge often still has BULK_UPLOAD_TEMP_DISK=local, which kept writing to per-node disk → Import: "Uploaded file no longer available".
• Outside local/testing, always use the cloud disk (s3) even if env/cache says local.
• Store via Storage::put (private), fail fast if the object is missing right after upload.
• Error message now includes the disk name for easier diagnosis.

Test plan

• Deploy, then upload a fresh file (old sessions still on local will not work)
• Preview → Import should succeed
• Optional: set Forge BULK_UPLOAD_TEMP_DISK=s3 and config:clear (code no longer requires this, but keeps config honest)
• If it still fails, note whether the error says (disk: s3) or (disk: local)

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