Azure.Sdk.Tools.TestProxy_1.0.0-dev.20260521.2

Use POSIX file semantics on Windows for the breadcrumb (#15704)

* Use POSIX file semantics on Windows for the breadcrumb

Concurrent test proxy instances running against the same
--storage-location intermittently fail on Windows due to a file sharing
violation on the breadcrumb.  This is because the APIs in use have
restrictive file share modes on Windows that doesn't exist on other
platforms.
This change relaxes the share mode on Windows so it matches the behavior
on other platforms.

* use async FileStream