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)

Motivation and Context (Optional)

Related Issue (Optional)

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• I have reviewed the AI generated code thoroughly.
• I possess the technical expertise to responsibly review the code generated in this PR.

[acud]

this package again... why not synctest and some other channel driven mocks or something? would really like to see this package go away from the codebase

[acud]

i would test here the method that creates the set of peers to be sent to every peer separately. just unit test that the sets are correct. then calling discovery is either trivial (and can go without a test at all) or very simple to test.