fix(eth/downloader): include xdc164 peers in idle peer selection

[gzliudan]

Proposed changes

This PR fixes the bug: "no peers available or all tried for download" which is introduced in #2470 and cause network stuck.

The xdc/164 protocol (EIP-2364 handshake) was added without updating the idlePeers version ceiling, which was still capped at 101 from the xdc100 era. Once all peers negotiate version 164, HeaderIdlePeers, BodyIdlePeers, ReceiptIdlePeers and NodeDataIdlePeers return empty sets, so fetchParts aborts every skeleton/body/receipt/state fetch with errPeersUnavailable (surfacing as "retrieved hash chain is invalid: no peers available or all tried for download"). Any node falling more than a few blocks behind can then never sync again.

Replace the hardcoded ceilings with a maxProtocolVersion constant covering xdc164 and add a regression test asserting that peers of every supported protocol version are eligible for concurrent downloads.

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• Consensus
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• Geth
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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.
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• 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

Fixes a downloader peer-selection regression where the idle peer selectors (used for concurrent header/body/receipt/state downloads) could exclude newer protocol versions (xdc/164), causing sync to stall with errPeersUnavailable once the network upgrades.

Changes:

• Introduce a maxProtocolVersion ceiling (set to 164) and use it in idle peer selection.
• Expand HeaderIdlePeers, BodyIdlePeers, ReceiptIdlePeers, and NodeDataIdlePeers to include xdc/164 peers.
• Add a regression test to ensure peers across supported protocol versions are considered eligible for idle selection.

Reviewed changes

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

File	Description
eth/downloader/peer.go	Replaces hardcoded protocol version ceilings in idle peer selection with a single maxProtocolVersion constant.
eth/downloader/peer_test.go	Adds a regression test ensuring idle peer selectors return peers across protocol versions.

Comments suppressed due to low confidence (1)

eth/downloader/peer.go:498

• BodyIdlePeers now filters out eth/62 peers by setting minProtocol=63, even though body fetching supports v62 (FetchBodies checks p.version < 62). Keep the lower bound at 62 to avoid needlessly excluding compatible peers.

	return ps.idlePeers(63, maxProtocolVersion, idle, throughput)