Eliminate AbstractRLPInput abstraction to reduce technical debt

[jflo]

Technical Debt Reduction: AbstractRLPInput/BytesValueRLPInput

Problem

Brittleness Signal: Unnecessary abstraction layer between AbstractRLPInput and BytesValueRLPInput where the abstract class has only one concrete implementation

Root Cause

Hidden Coupling: AbstractRLPInput (607 lines) contains all RLP parsing logic, while BytesValueRLPInput (84 lines) only provides 8 abstract method implementations for byte access
Files Involved:

• /ethereum/rlp/src/main/java/org/hyperledger/besu/ethereum/rlp/AbstractRLPInput.java:33
• /ethereum/rlp/src/main/java/org/hyperledger/besu/ethereum/rlp/BytesValueRLPInput.java:24

Why It Exists: Originally designed to support multiple RLP input sources, but only Bytes-based input was ever implemented

Proposed Solution

Approach: Merge AbstractRLPInput and BytesValueRLPInput into a single concrete BytesValueRLPInput class
Scope:

• Inline all abstract methods from AbstractRLPInput into BytesValueRLPInput
• Update RLP.input() factory method
• Remove AbstractRLPInput class

Success Criteria:

• Single concrete class replaces 2-class hierarchy
• All 84 direct instantiations continue working
• 567 total RLPInput usages unaffected (interface unchanged)

Implementation Plan

Step 1: Merge classes without changing public API

• Copy all concrete methods from AbstractRLPInput to BytesValueRLPInput
• Inline the 8 abstract method implementations
• Make all previously protected members private
• Run full test suite to verify functionality

Step 2: Update references and clean up

• Update RLP.input() methods to directly instantiate merged class
• Remove AbstractRLPInput.java file
• Update import statements if needed
• Verify no compilation errors

Safety Checks

• Can revert each step independently
• Tests exist to verify RLP functionality is unchanged (ethereum/rlp test suite)
• No changes to public APIs (RLPInput interface remains unchanged)
• Change only affects ethereum/rlp module

Blast Radius Assessment

Direct Dependencies: 2 files in ethereum/rlp module only
Hidden Connections: 84 direct instantiations across codebase continue to work via unchanged constructor
Transitive Impact: Zero - RLPInput interface contract preserved
Risk Level: Small (confined to single module, no API changes)

Definition of Done

• Original two-class hierarchy replaced with single class
• All existing tests pass without modification
• Code reviewer confirms abstraction complexity removed
• No functionality changes or new surprise failures

Additional Context

This follows the pattern identified where abstract classes with single implementations indicate over-engineering. The RLP module is mature and stable, making this an ideal candidate for simplification.