bifrost: emit reorg errata only on definitive not-found

Summary

This MR hardens reorg errata handling so we only emit errata when tx absence is definitive, and avoid false errata on transient RPC/node failures.

What changed

• Added shared reorg helpers in bifrost/pkg/chainclients/shared/reorg:
  • TxCheckStatus (Confirmed, Errata, Unknown)
  • IsUTXOTxNotFoundErr
  • IsUTXOTxNotInMempoolErr
  • IsEVMTxNotFoundErr
• Refactored chain clients to use shared status/helpers:
  • bitcoin/client.go
  • ethereum/ethereum_block_scanner.go
  • evm/evm_block_scanner.go
  • shared/evm/eth_rpc.go
• Behavior change:
  • RPC/network/timeout/non-definitive errors -> Unknown (log warning, keep tx in metadata, no errata)
  • definitive not-found -> Errata
• Added shared tests:
  • bifrost/pkg/chainclients/shared/reorg/reorg_test.go

Why

Original issue: tx reconfirmation could treat any RPC failure as “missing tx” and emit errata incorrectly.
This MR centralizes classification and makes errata emission explicit and conservative.

Summary by CodeRabbit

• Improvements
  • More nuanced transaction-status detection during reorganizations, distinguishing confirmed, missing (errata), and uncertain cases for Bitcoin, Zcash, and EVM chains.
  • Enhanced mempool checks to reduce false errata reports and improve confirmation accuracy.
  • Clearer logging and more robust handling of transient RPC errors, yielding fewer mistaken removals of valid transactions.