fix(p2p/discover): return nil when v4 resolve cannot refresh node

[gzliudan]

Proposed changes

Restore Resolve failure semantics to return nil so dial backoff can detect unresolved nodes correctly.

Add a regression test covering unresolved ENR refresh paths.

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[Copilot]

Pull request overview

This PR restores expected endpoint-resolution semantics in Discovery v4 by making UDPv4.Resolve return nil when a node cannot be resolved, allowing the dial scheduler’s backoff logic to correctly treat unresolved nodes as failures.

Changes:

• Change UDPv4.Resolve to return nil (instead of returning the input node) when resolution fails, including the “no secp256k1 key” case.
• Update the Resolve doc comment to match the new behavior.
• Add a regression test asserting Resolve returns nil for an unresolved node.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
p2p/discover/v4_udp.go	Restores Resolve failure semantics to return nil so dial backoff can detect unresolved nodes.
p2p/discover/v4_udp_test.go	Adds a regression test for Resolve returning nil on an unresolved node path.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.