feat(flashblocks): Decompose `build_pending_state` into Testable Components

[refcell]

Summary

Break the large build_pending_state method into distinct, unit-testable components.

Problem

StateProcessor::build_pending_state() conflates 4 concerns:

1. Block reconstruction from flashblocks
2. EVM context initialization
3. Parallel sender recovery
4. Sequential transaction execution

This makes it impossible to test any piece in isolation.

Solution

Extract into separate components:

1. BlockAssembler - Reconstructs blocks from flashblocks

pub struct AssembledBlock {
    pub block: OpBlock,
    pub base: FlashblockBase,
    pub flashblocks: Vec<Flashblock>,
    pub header: Sealed<Header>,
}

impl BlockAssembler {
    pub fn assemble(flashblocks: &[&Flashblock]) -> eyre::Result<AssembledBlock>;
}

2. SenderRecoveryService - Parallel ECDSA with caching

impl SenderRecoveryService {
    pub fn recover_all(
        transactions: impl IntoIterator<Item = &OpTxEnvelope>,
        cached_senders: &HashMap<B256, Address>,
    ) -> eyre::Result<Vec<(OpTxEnvelope, Address)>>;
}

3. BlockExecutor trait - Prep for future EVM mocking

pub trait BlockExecutor {
    fn execute_transaction(
        &mut self,
        index: usize,
        transaction: Recovered<OpTxEnvelope>,
    ) -> eyre::Result<ExecutedTransaction>;
}

Refactored build_pending_state

fn build_pending_state(&self, prev: Option<Arc<PendingBlocks>>, flashblocks: &[Flashblock]) -> Result<...> {
    let grouped = Self::group_by_block_number(flashblocks);
    let mut builder = PendingBlocksBuilder::new();

    for (_block_num, fbs) in grouped {
        let assembled = BlockAssembler::assemble(&fbs)?;
        let txs_with_senders = SenderRecoveryService::recover_all(&assembled.block.body.transactions, &cached)?;
        let executed = self.execute_block(&assembled, txs_with_senders, ...)?;

        for tx in executed {
            builder.with_executed_transaction(tx.rpc_tx, tx.receipt, tx.evm_state, tx.sender);
        }
    }

    Ok(Some(Arc::new(builder.build()?)))
}

Acceptance

• BlockAssembler unit tests (single/multiple flashblocks, missing base)
• SenderRecoveryService unit tests (cache hits, cache misses)
• Integration tests still pass

[PoulavBhowmick03]

@refcell can I take this up?