MediaAttacher::download() only caught RuntimeException, but a connection-level
failure throws Illuminate\Http\Client\ConnectionException (extends Exception, not
RuntimeException) — e.g. a slow/unreachable proxy hitting the 20s timeout. On the
API hosting path that propagated as a 500 (not the promised 422), skipped the
batch rollback (orphaning an already-hosted item), and leaked the temp file.
Catch Throwable so any fetch failure returns null → the caller rejects cleanly
with 422 and rolls back. Also hardens the existing MCP/REST attach-from-url paths
(a timeout there now reports a failed URL instead of 500).
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