feat(primitives-traits): add try_recover_signers for parallel batch recovery

[mattsse]

Summary

Add try_recover_signers function that combines decoding and signature recovery in a single parallel pass. This is more efficient than sequential recovery for large transaction batches.

Changes

• Add try_recover_signers to transaction::recover module
• Uses rayon for parallel recovery when the rayon feature is enabled
• Update testing RPC to use batch recovery instead of sequential

Usage

let recovered_txs = try_recover_signers(&raw_transactions, |tx| {
    MyTxType::decode_2718_exact(tx.as_ref())
        .map_err(RecoveryError::from_source)
})?;

[gakonst]

Nice refactor! The new try_recover_signers API is a clean abstraction for combining decode + recovery in a single parallel pass.

Code review:

1. Flat module structure - Good call eliminating the nested rayon/iter modules. The #[cfg] dispatch at the function level is cleaner.

2. Trait bounds - The TryRecoverItems and TryRecoverFn helper traits handle the Sync requirement for rayon nicely. The blanket impls keep usage ergonomic.

3. RPC integration - The testing.rs change demonstrates the API well. Replacing the sequential recover_raw_transaction loop with try_recover_signers is a nice win for batch operations.

4. Error handling - Using .or(Err(EthApiError::InvalidTransactionSignature)) loses the original error context. Consider using .map_err(|_| ...) to at least log the underlying error, but this is minor for testing endpoints.

LGTM