perf(core/txpool/legacypool): use maps.Keys and maps.Copy #30091

[gzliudan]

Proposed changes

• slice and map are reference types, we can use themselves as pointers.
• Use maps.Keys and maps.Copy simplify code.

Ref: ethereum#30091

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• chore: Changes that don't change source code or tests
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• test: Adding missing tests or correcting existing tests

Impacted Components

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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
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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
• N/A

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[Copilot]

Pull request overview

This PR updates core/txpool/legacypool to use Go’s maps helpers to simplify set flattening and merging logic inside the legacy transaction pool.

Changes:

• Replace manual key-collection loop with maps.Keys + slices.Collect in accountSet.flatten.
• Replace manual merge loop with maps.Copy in accountSet.merge.

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[Copilot]

flatten previously always returned a non-nil empty slice when the set was empty (due to make([]T, 0, ...)). With slices.Collect(maps.Keys(...)), the result will be nil when as.accounts is empty, which can be an observable behavior change (e.g., JSON encoding null vs [], or callers comparing to []common.Address{}). Consider ensuring accounts is initialized to an empty (non-nil) slice when there are zero keys to preserve the previous behavior.

Suggested change

	accounts := slices.Collect(maps.Keys(as.accounts))
	accounts := slices.Collect(maps.Keys(as.accounts))
	if accounts == nil {
	accounts = make([]common.Address, 0)
	}