blockchain: isolate tx cache during prefetch

[yoomee1313]

Summary

• During mainnet sync, one block was rejected once and later imported successfully.
• GasUsed mismatch was 15000 (134249803 remote vs 134234803 local).
• The delta matches one Kaia signature-validation key.
• The peer was v2.2.2, so logs/code-version differences made the symptom hard to isolate.

Cause

• EVM-based prefetch calls tx.AsMessageWithAccountKeyPicker(...).
• That method mutates tx wrapper caches, including validatedGas.
• Prefetch used the original block tx pointer.
• Speculative prefetch state could overwrite caches later used by real insertChain execution.
• The added unit test reproduces the 15000 gas cache pollution case.

Fix

• Use a fresh transaction wrapper before prefetch execution:

types.NewTx(tx.GetTxInternalData())

• This keeps original block transaction caches untouched.
• It does not deep-copy tx data, signatures, or payload.

History

• 5e25a0f71: added copied tx prefetching.
• c57ec37bd: removed copied tx logic.
• Later locks prevent data races, but not logical cache pollution from speculative prefetch.
• kaiabft avoids this differently via d4671fa8a, but that is a larger behavior/performance change, so this PR keeps the fix minimal for dev.

Scope

• Affected:
  • block import/sync through insertChain prefetch paths
• Not affected:
  • proposer block creation path
  • proposer path does not use statePrefetcher, Prefetch, or PrefetchTx

Test

• Before the fix, the regression test fails because prefetch overwrites the original tx gas cache.
• After the fix, the same regression test passes and the full blockchain package test passes.

# Before the fix: copyTxForPrefetch returns the original tx.
$ go test ./blockchain -run TestCopyTxForPrefetchDoesNotOverwriteOriginalValidatedGas -count=1
INFO[08/06,17:37:04 +08] [5] InitDeriveSha                             initial=0 withGov=false
--- FAIL: TestCopyTxForPrefetchDoesNotOverwriteOriginalValidatedGas (0.00s)
    state_prefetcher_test.go:92:
        Error Trace:    /Users/yumiel/.codex/worktrees/6174/kaia/blockchain/state_prefetcher_test.go:92
        Error:          Not equal:
                        expected: 0x8ca0
                        actual  : 0x5208
        Test:           TestCopyTxForPrefetchDoesNotOverwriteOriginalValidatedGas
FAIL
FAIL    github.com/kaiachain/kaia/blockchain    0.549s
FAIL

# After the fix.
$ go test ./blockchain -run TestCopyTxForPrefetchDoesNotOverwriteOriginalValidatedGas -count=1
ok      github.com/kaiachain/kaia/blockchain    0.547s

$ go test ./blockchain -count=1
ok      github.com/kaiachain/kaia/blockchain    20.236s