rpc : do not leave or serve truncated cache files - #30648

Overview

When rpc-server -c fails to write a cache file (e.g. disk full), it leaves a truncated or empty file under the cache dir. On later requests get_cached_file treats that file as a valid hit and serves the truncated data, which silently corrupts weights.

This PR:

• writes cache files to <hash>.tmp first and renames to <hash> only after a successful write; on failure the temp file is removed and a warning is logged
• treats empty or unreadable cache files as a cache miss in get_cached_file, so the client falls back to sending the full tensor

Fixes #30633

Additional information

test-rpc-multi-server now starts server A with -c and a temp LLAMA_CACHE, pre-creates an empty cache file for a weights tensor, and checks that set_tensor still round-trips the correct data and that no empty or .tmp files are left in the cache dir afterwards.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO