chain: Fix TxPool race

[lukechampine]

The txpool should never expose raw transactions in the pool without deep-copying them. This was causing a data race when a parent transaction (returned by V2TransactionSet) was later mutated by applyPoolUpdate.

Fixes #261

[Copilot]

Pull Request Overview

This PR addresses a data race by ensuring transactions returned from the pool are deep-copied rather than exposing the originals.

• Use slices.Clone for PoolTransactions to copy the slice header.
• Apply DeepCopy() to v2 transactions in TransactionsForPartialBlock and V2TransactionSet.
• Remove a stale TODO.

Comments suppressed due to low confidence (3)

chain/manager.go:880

• TransactionsForPartialBlock still returns raw pool transactions without deep-copy, which can reintroduce races. Consider using txn.DeepCopy() when appending to txns.

for _, txn := range m.txpool.txns {

chain/manager.go:838

• Add unit tests for PoolTransactions, TransactionsForPartialBlock, and V2TransactionSet that verify modifying returned transactions doesn’t affect the pool, preventing future race regressions.

func (m *Manager) PoolTransactions() []types.Transaction {

chain/manager.go:841

• Ensure the slices package is imported (import "slices") and confirm the project’s minimum Go version is 1.21 or newer to support slices.Clone.

return slices.Clone(m.txpool.txns)