fix(bootstrap): restore periodic rounds when no backup peers are set

[guillaumemichel]

Problem

Since #710, bootstrap.Bootstrap only behaves as documented when backup peers are set with WithBackupPeers or SetBackupPeers. Without them:

• Periodic rounds never run: after the first round, the supervisor goroutine waits on a channel that only the backup-peers path sends to. A node that later drops below MinPeerThreshold connections never gets another bootstrap round.
• The supervisor goroutine leaks: it is blocked on that channel rather than on the context, so Close never releases it.
• Bootstrap returns before the first round completes: while with backup peers it waits for that round.

#710 moved the first-round handshake (doneWithRound <- struct{}{}) inside if cfg.loadBackupBootstrapPeers != nil and dropped the unconditional close(doneWithRound). Without backup peers, nothing ever sends on or closes that channel. Before #710, every caller waited for the first round and got periodic rounds

Kubo always sets backup peers, so it isn't affected.

Fix

• The supervisor closes firstRoundDone once the first round completes, then enters the periodic loop. It no longer waits on a handshake.
• Bootstrap waits on firstRoundDone whether or not backup peers are set, then starts the backup-saving process when they are.
• When rt.Bootstrap fails, Bootstrap cancels the context and returns the error, as before. The supervisor's first round ends early on the cancelled context, then the supervisor closes the channel and exits, so there is nothing left to unblock.

For callers without backup peers, two things change, both back to how they were before #710:

• Bootstrap waits for the first round, which can take up to ConnectionTimeout.
• Every Period, a round checks the connection count, and dials bootstrap peers only when it is below MinPeerThreshold. Callers with backup peers already get exactly this.

Callers with backup peers see no change, and the backup-peer API is unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.73%. Comparing base (3d6ac39) to head (46d947f).

@@            Coverage Diff             @@
##             main    #1220      +/-   ##
==========================================
+ Coverage   70.54%   70.73%   +0.18%     
==========================================
  Files         270      270              
  Lines       27265    27264       -1     
==========================================
+ Hits        19234    19285      +51     
+ Misses       7984     7932      -52     
  Partials       47       47              

Files with missing lines	Coverage Δ
bootstrap/bootstrap.go	81.62% <100.00%> (+27.85%)	⬆️

... and 4 files with indirect coverage changes

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[gammazero]

Good fix. Yes, looks like moving doneWithRound <- struct{}{} inside if cfg.loadBackupBootstrapPeers != nil was an error.