feat(core/txpool,eth): track already-known local transactions on resubmit

[gzliudan]

Summary

A local transaction that reaches the pool through a concurrent submission
(peer gossip, wallet retry, or a parallel RPC endpoint) returns
txpool.ErrAlreadyKnown from TxPool.Add, but is not placed in the desired
local-tracking state: the local tracker only starts on a successful admission.
If that transaction is later evicted, it loses its local resubmit and journal
protection and silently disappears.

This PR treats ErrAlreadyKnown as the desired state on the initial admission
path. AddLocal and EthAPIBackend.SendTx still surface the error to the
caller (matching upstream go-ethereum semantics) but now also register the
transaction with the local tracker so it keeps the resubmit and journal
guarantees.

Background

The resubmit loop (TxTracker.loop -> TxTracker.recheck) already retains
already-known transactions in the tracked set, because recheck skips
transactions still present in the pool (pool.Has). The only gap was on the
initial admission path, which this PR closes.

Changes

• core/txpool/txpool.go: AddLocal tracks the transaction when it is
  accepted, temporarily rejected, or already known to the pool.
• eth/api_backend.go: SendTx tracks the transaction unless it is
  permanently rejected; an already-known transaction is still tracked while
  the error is surfaced to the caller.
• Tests: TestAddLocalTracksAlreadyKnownTransaction,
  TestSendTxTracksAlreadyKnown.

Test plan

• go test ./core/txpool/ -run TestAddLocal
• go test ./eth/ -run TestSendTx

Risk

Low. The change only adds tracking for an already-known transaction and
preserves the original error returned to the caller; the resubmit path was
already consistent.

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[Copilot]

Pull request overview

Ensures already-known local transactions retain resubmission and journal protection while preserving the original caller error.

Changes:

• Tracks ErrAlreadyKnown transactions in both local submission paths.
• Adds focused unit and integration tests for the behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
core/txpool/txpool.go	Tracks already-known transactions in AddLocal.
core/txpool/txpool_local_test.go	Tests AddLocal tracking and error behavior.
eth/api_backend.go	Tracks already-known RPC submissions while returning the error.
eth/api_backend_test.go	Tests the RPC submission path.

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