Bound blob downloads with per-source timeouts

[ajpallares]

Motivation

Blob downloads had no per-request connect/read timeout, so a slow or failing blob source stretched getOfferings latency across the source-failover loop.

Description

• Bound each blob-source attempt via Re-tier HTTP request timeouts around per-host memory #3746's shared timeout manager and its no-fallback tier (15s base, 5s if that host timed out in the last 10 min), keyed by the blob source host.
• Record per-host results so a flaky source fails fast on re-hit; a timeout still advances to the next source as before.

Stacks on #3746.