refactor(libp2p): use precise connection cleanup in handshake error handling

[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

refactor(libp2p): use precise connection cleanup in handshake error handling

Replace ClosePeer() with Conn().Close() for connection-specific errors
during handshake. ClosePeer() terminates all connections to a peer,
while Conn().Close() only closes the specific failed connection.

This prevents disrupting healthy concurrent connections and improves
handling of duplicate connection attempts by preserving existing
sessions when rejecting new redundant connections.

Changes:

• Use stream.Conn().Close() instead of ClosePeer() in handshake error paths
• Improve duplicate peer handling to only close the new connection
• Add clarifying comment for duplicate connection scenario

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

Screenshots (if appropriate):

[janos]

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