refactor(p2p/discover): move discv4 encoding to new 'v4wire' package #21147

[gzliudan]

Proposed changes

This moves all v4 protocol definitions to a new package, p2p/discover/v4wire. The new package will be used for low-level protocol tests.

Ref: ethereum#21147

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• refactor: A code change that neither fixes a bug nor adds a feature
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• test: Adding missing tests or correcting existing tests

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Checklist

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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
• Tested on a private network from the genesis block and monitored the chain operating correctly for multiple epochs.
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• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
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[Copilot]

Pull request overview

Refactors Discovery v4 by extracting wire-format packet types and (en|de)coding into a new p2p/discover/v4wire package, enabling low-level protocol testing independently from the UDP transport implementation.

Changes:

• Add p2p/discover/v4wire package containing v4 packet structs, constants, and Encode/Decode helpers.
• Update p2p/discover/v4_udp.go to use v4wire types and encoding/decoding, replacing inline wire logic with handler wrappers.
• Move EIP-8 forward-compatibility test vectors into v4wire tests and adjust existing UDP/lookup tests to use v4wire.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
p2p/discover/v4wire/v4wire.go	New v4 wire protocol types and encode/decode utilities extracted from UDPv4.
p2p/discover/v4wire/v4wire_test.go	New low-level EIP-8 forward-compatibility vectors and decode assertions for the wire package.
p2p/discover/v4_udp.go	Reworks UDPv4 to rely on v4wire for packet framing and to dispatch via wrapper handlers.
p2p/discover/v4_udp_test.go	Updates UDP tests to use v4wire packets/codec and removes duplicated EIP-8 vectors from UDP tests.
p2p/discover/v4_lookup_test.go	Updates lookup test harness to operate on v4wire.Packet and v4wire.Node.
p2p/discover/node.go	Removes recoverNodeKey from discover package (now provided by v4wire).