fix(core,core/rawdb): clean up XDPoS snapshot on SetHead

[gzliudan]

Proposed changes

Restrict SetHead snapshot cleanup to block numbers that can persist XDPoS snapshots based on epoch/gap rules.

Keep unified deletion semantics (V2 first, then V1 fallback) and align SetHead tests with XDPoS gating.

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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

This PR tightens SetHead’s XDPoS snapshot cleanup so it only targets block heights that are eligible to have persisted XDPoS snapshots (based on Epoch/Gap), and adds a rawdb helper plus a SetHead-focused unit test to reflect the new behavior.

Changes:

• Add rawdb.DeleteXdposSnapshot with “V2 first, then V1 fallback” semantics.
• Gate snapshot deletion during BlockChain rewind to heights satisfying (num + gap) % epoch == 0.
• Add a SetHead test that exercises V2 deletion and V1 fallback deletion paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
core/rawdb/accessors_xdc.go	Adds XDPoS snapshot deletion helpers and an exported delete entrypoint.
core/blockchain.go	Gates XDPoS snapshot cleanup during SetHead rewinds using epoch/gap rules.
core/blockchain_sethead_test.go	Adds a SetHead unit test covering XDPoS snapshot cleanup behavior.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.