Remove use of Linq.Sum from hot paths

[benaadams]

Changes

• Use simple for loop rather than allocating a closure and ienumerable
• Add .Count property for StorageKeys so don't need to enumerate them to work out count

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• No

[Copilot]

Pull Request Overview

This PR optimizes hot paths by removing LINQ allocations and replacing them with explicit loops, while also exposing a Count property on StorageKeysEnumerable to avoid unnecessary enumeration.

• Replace Sum calls with manual loops in various RLP serialization decoder methods.
• Add Count property to StorageKeysEnumerable to use direct length rather than Count() enumeration.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
AccessListDecoder.cs	Replaced Select(...).Sum(...) with a manual loop in GetContentLength and switched Count() to Count property.
BlockBodyDecoder.cs	Replaced array Sum calls in GetTxLength, GetUnclesLength, and GetWithdrawalsLength with explicit foreach loops.
ValidateBuilderSubmissionHandler.cs	Replaced Sum on transactions’ blob arrays with a loop to calculate totalBlobsLength.
AccessList.cs	Added a public Count property to StorageKeysEnumerable.
PendingValidatorsDecoder.cs	Replaced Sum on address arrays with an explicit loop in GetAddressesLength.

Comments suppressed due to low confidence (1)

src/Nethermind/Nethermind.Serialization.Rlp/Eip2930/AccessListDecoder.cs:199

• The new manual calculation in GetContentLength introduces non-trivial logic. Adding a unit test to verify this method returns the same value as the previous LINQ-based implementation would help ensure correctness.

        private static int GetContentLength(AccessList accessList)