refactor(txpool/legacypool): remove ErrAlreadyKnown in legacypool #28400

[gzliudan]

Proposed changes

deprecated ErrAlreadyKnown in legacypool

Ref: ethereum#28400

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[Copilot]

Pull request overview

This PR refactors the legacy transaction pool to stop defining its own ErrAlreadyKnown sentinel error and instead reuse the shared txpool.ErrAlreadyKnown error.

Changes:

• Removed legacypool.ErrAlreadyKnown from core/txpool/legacypool/legacypool.go.
• Updated code paths that reported “already known” to return/assign txpool.ErrAlreadyKnown.

Comments suppressed due to low confidence (1)

core/txpool/legacypool/legacypool.go:66

• PR description says “deprecated ErrAlreadyKnown in legacypool”, but this change removes the exported identifier entirely (API breaking for any downstream imports of legacypool). If the intent is deprecation, keep a deprecated alias (e.g., ErrAlreadyKnown pointing to txpool.ErrAlreadyKnown) for a release or two; otherwise, please update the PR description / release notes to reflect that it’s removed.

var (
	// ErrTxPoolOverflow is returned if the transaction pool is full and can't accept
	// another remote transaction.
	ErrTxPoolOverflow = errors.New("txpool is full")

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