logging: tag serving Esplora host on success, not just failover

[LandynDev]

What

Adds one debug line in the Esplora request chokepoint (btc_api_request) so the serving host is logged on the normal first-provider success, not only on failover/error.

Before #364's narration, a healthy validator hitting its primary Esplora backend logged nothing — you couldn't confirm which backend was actually serving without waiting for a failover. This makes the active provider visible on the success path too.

Change

elif i > 0:
    bt.logging.info(... served after {i} fallback(s))
else:
    bt.logging.debug(f'Esplora {pos} {tag}{path} → {resp.status_code}')   # new

• Single additive branch, no behavior change.
• Logged at debug so it's visible on validators run with --logging.debug (where you actually want this confirmation) without spamming info-level deployments on every tip-height poll.
• Covers all critical reads, since swap verification, tx_exists, find_recent_outgoing, balance, and broadcast all route through btc_api_request.

Failover (warning) and fallback-served (info) lines are unchanged.