telemetry: keep known peers when a discovery refresh fails

[elitegreg]

Resolves: #4128

Independent of #4144 and #4145 (different file), so this one branches from main.

Summary of Changes

• ledgerPeerDiscovery.refresh no longer empties the peer cache before doing work that can fail. It cleared p.peers under the lock and then called LocalNet.Interfaces(), so a transient failure there returned with zero peers and Pinger.Tick iterated an empty slice, probing nothing until a later refresh succeeded.
• The cache is now replaced only once the new list is built, and the lock covers just that assignment rather than the whole build. The clear was redundant with the existing assignment at the end of the happy path.
• Success path is unchanged.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Tests	1	+79 / -0	+79
Core logic	1	+6 / -5	+1
Docs	1	+3 / -0	+3
Total	3	+88 / -5	+83

A one-line behavioral fix plus the regression test that pins it.

Key files (click to expand)

• controlplane/telemetry/internal/telemetry/peers.go — drops the p.peers = make(...) clear, moves the mutex to wrap only p.peers = peers, and leaves a comment that nothing in the build may clear the cache

Testing Verification

• New test lets the first refresh discover a peer, then fails every subsequent LocalNet.Interfaces() call, and asserts GetPeers() still returns the fully populated peer (link, device, tunnel, TWAMP port) after at least three failed refreshes. It fails against the pre-fix code, which returns an empty list.
• Existing peer discovery tests pass unchanged, covering the success path and the skip cases.
• Package passes under -race, since the change moves what the mutex covers.