Fix GCS Mock Range Downloads

[original-brownbear]

We were not correctly respecting the download range which lead
to the GCS SDK client closing the connection at times.

Closes #51446

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)

[original-brownbear]

@tlrx I'm not 100% positive this fixes the issue that caused the last #51446 failures but it's the only thing I could find going wrong at the moment that causes connection drops.
(I could reproduce the failure in the issue once after O(10k) iterations and can't any more with this fix so I'm somewhat optimistic)
If anything it should stabilize the behavior of the mock http server because it removes a source of blocking in general because it does its writes in a blocking fashion (sets the client sockets to blocking before writing) and this change makes it so we never write more than the buffer size making the IO loop block a lot less.

[original-brownbear]

Bam random find :) => This might actually be the culprit in the linked issue e6565ee :)

I'd still like to fix the other issue in here as well.

[tlrx]

LGTM, thanks for persevering on this Armin!

[original-brownbear]

Np + thanks for reviewing Tanguy!