fix: include registered extra props in PROPFIND request body

[flash7777]

Summary

registerExtraProp() registered properties for response parsing but buildPropFindBody() never included them in the PROPFIND request XML. Extensions using registerExtraProp() would never receive their custom properties because the server was never asked for them.

Replaces #2721 (which had unrelated files).

Changes

File	Change
builders.ts	Include registered extra props in PROPFIND XML body

Test plan

• Extension registers extra prop via registerExtraProp('oc:my.prop')
• PROPFIND request XML now includes <oc:my.prop/>
• Server returns the property value in response
• Existing props (etag, fileid etc.) still work