fix(kademlia): neighborhood rebroadcast sends wrong peer lists

[gacevicljubisa]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

The 15-minute neighborhood rebroadcast in the kademlia manage loop builds each
neighbor's list with append(neighbors[:i], neighbors[i+1:]...). That appends
into the backing array of the slice the loop is still ranging over, so from the
second iteration on the loop reads already-overwritten entries. With four
neighbors [a b c d] the loop actually sends:

i=0 -> a: [b c d] (neighbors is now [b c d d])
i=1 -> c: [b d d]
i=2 -> d: [b d d]
i=3 -> d: [b d d]

Only the first neighbor gets a correct list, b is never told anything, d is
told twice and told about itself, and the lists carry duplicates. Present since
#4412.

Changes:

• Build the list with slices.Concat, which allocates a fresh slice.
• Move the loop body into rebroadcastNeighborhood and expose it from
  export_test.go, so it can be driven without waiting fifteen minutes.
• Add TestRebroadcastNeighborhood: connects four peers with storage radius 0,
  waits for the connect-time announces to settle, runs one rebroadcast, and
  asserts each neighbor receives exactly one message listing the other three
  and never itself. The test fails on master with "received 6 peers, want 3".

Open API Spec Version Changes (if applicable)

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