server: refactor subproc handling (ggml-org#28555)

* server: refactor subproc handling

* fix Windows build

* download: keep concurrent downloads of one blob apart

Every process writes the same path + .downloadInProgress, so a second
download of the same blob finds that file, takes it for its own partial
transfer and asks for the bytes after it, which produces a corrupt
result. The in-progress file now carries the pid of the process writing
it.

std::rename also replaces an existing destination on POSIX but fails on
Windows, so a download whose blob appeared in the meantime is dropped
after every retry and an etag rewrite silently keeps the old value.
std::filesystem::rename has the POSIX behaviour everywhere, and the
error now carries the reason reported by the system.

* Revert "download: keep concurrent downloads of one blob apart"

This reverts commit 917b83f.

* tests: serialize the router tests that download the same model

Parallel workers share one cache, so the two tests fetch the same blob
into the same in-progress file and race to rename it. They now take a
file lock around the download, like the session fixture does for the
preset models.

* Revert "tests: serialize the router tests that download the same model"

This reverts commit c368a4a.

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Co-authored-by: Pascal <[email protected]>