fix(p2p): don't discard reason sent by Disconnect() #16559

[gzliudan]

Proposed changes

Ref: ethereum#16559

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
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[Copilot]

Pull request overview

This PR fixes a bug where the disconnect reason passed to Peer.Disconnect() was being discarded in the peer's run loop. When Disconnect() is called with a specific reason, that reason is sent through the p.disc channel, but the run loop was not extracting this reason before closing the connection, resulting in a default reason being used instead.

Changes:

• Added missing reason = discReasonForError(err) assignment when receiving from p.disc channel to properly handle the disconnect reason sent by Disconnect()

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