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

Types of changes

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• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Changes that don't change source code or tests
• docs: Documentation only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that neither fixes a bug nor adds a feature
• revert: Revert something
• style: Changes that do not affect the meaning of the code
• test: Adding missing tests or correcting existing tests

Impacted Components

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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

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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