test(duties_tracker): cover poll_beacon_proposers fetch path via a beacon-node seam

[petarjuki7]

Deferred from #1183.

ProposerSchedule::from_response is fully unit-tested, but nothing drives poll_beacon_proposers itself: the crate has no way to fake a beacon-node response (BeaconNodeFallback is constructed over a real HTTP client).

Untested as a result:

• the fetch uses get_validator_duties_proposer_v2 (not v1)
• a failed fetch leaves the previously stored schedule in place
• a malformed response is rejected at the poll site and leaves the previously stored schedule in place

Needs either a mock HTTP server for the fallback client or a fetch abstraction the tests can substitute. Same shape as #1238.