Add network transfer regression test for #942

[julienmoumne]

This is the first part of addressing #942.

It adds automated simulation coverage for the network traffic that continues after XetFileDownloadGroup::abort(), with sigint_abort() as a positive control.

It leverages the existing simulation infrastructure, adding a total_download_transferred counter to observe bytes actually forwarded to the client.

The failing regression is ignored for now so CI stays green.

Use this to run it :

cargo +1.94.1 test -p hf-xet --lib --features simulation test_group_abort_stops_network_transfer -- --include-ignored --nocapture

The next step is to fix the cancellation propagation and enable the regression test.

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Note

Low Risk
Changes are confined to simulation proxy metrics and gated integration tests; no production download or abort logic is modified yet.

Overview
Adds observable download bytes in the network simulation stack so tests can tell whether traffic keeps flowing after cancellation, as part of work on #942.

The bandwidth-limit proxy now tracks bytes actually forwarded upstream→client (not just theoretical refill capacity), plumbed through LocalTestServer::total_download_bytes_transferred(). Copy helpers take an optional counter updated on each written chunk.

New simulation tests in file_download_group start a throttled multi-range download, cancel mid-transfer, then assert the proxy byte counter stays flat. test_group_abort_stops_network_transfer is #[ignore] as the known failing regression; test_sigint_abort_stops_network_transfer runs as a positive control.

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