rpc: fix otterscan API stability and correctness issues

[lupin012]

This PR contains the following fixes

• Replace panic(err) with proper error handling in GetContractCreator and GetTransactionBySenderAndNonce
• Fix uint256 zero check using IsZero() instead of Uint64() != 0 in OperationsTracer
• Add missing defer it.Close() to prevent iterator resource leak
• Fix nil dereference in GetContractCreator: add tracer.Found() check before accessing tracer.Tx.Hash() to prevent panic when contract creator is not found
• Fix genericTracer returning nil error silently when header or txn is not found; now returns explicit error to prevent callers from proceeding with uninitialized tracer state
• Remove dead code: prunedBlock was computed but never used in GetBlockTransactions (present since PR Upstream all Otterscan patches to devel #5945)
• Fix OOB panic in GetBlockTransactions: add bounds check on receipts slice before slicing with page offsets
• Fix gas price truncation in delegateBlockFees: replace Uint64() intermediate with ToBig() to preserve full uint256 precision
• Fix SELFDESTRUCT handler in TransactionTracer: guard against empty Results slice to prevent index out of range panic
• Fix OnTxEnd call inconsistency: use tracer.Hooks.OnTxEnd consistently instead of mixing promoted field access with explicit field access