[Intermediate]: Support Jumbo EthereumTransaction (HIP-1086)

[0xivanov]

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👾 Description of the Issue

HIP-1086: Jumbo EthereumTransaction allows Ethereum transactions up to 128KB to be sent directly to the network without using the Hedera File Service (HFS).

The C++ SDK currently has a 5KB limit that triggers HFS usage for larger transactions:

// src/sdk/main/include/EthereumFlow.h, line 92
static constexpr unsigned int MAX_ETHEREUM_DATA_SIZE = 5120U;

With jumbo transactions now supported, users should just use EthereumTransaction directly, and EthereumFlow should be deprecated.

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💡 Proposed Solution

1. Update the Size Limit

Change MAX_ETHEREUM_DATA_SIZE from 5KB to 128KB:

// In EthereumFlow.h
static constexpr unsigned int MAX_ETHEREUM_DATA_SIZE = 128000U;  // 128KB jumbo limit

2. Deprecate EthereumFlow

Add deprecation notice to EthereumFlow class:

/**
 * @deprecated Use EthereumTransaction instead. With the introduction of jumbo 
 * transactions, it should always be less cost and more efficient to use 
 * EthereumTransaction instead.
 */
class [[deprecated("Use EthereumTransaction instead - jumbo transactions are more efficient")]] 
EthereumFlow
{
  // ...
};

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👩‍💻 Implementation Steps

1. Update constant: Change MAX_ETHEREUM_DATA_SIZE to 128000U in EthereumFlow.h

2. Add deprecation: Add [[deprecated]] attribute to EthereumFlow class

3. Update documentation: Add comment explaining why the class is deprecated

4. Add integration tests: Add tests for:

   • EthereumTransaction with large calldata (e.g., 120KB) - should succeed directly
   • EthereumFlow with calldata below 128KB - should succeed without HFS
   • EthereumFlow with calldata above 128KB - should use HFS fallback

5. Verify existing tests: Ensure existing tests still pass

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✅ Acceptance Criteria

To help get this change merged smoothly:

• Constant updated: MAX_ETHEREUM_DATA_SIZE changed to 128000U
• Deprecation added: EthereumFlow class marked deprecated with clear message
• Integration tests added: Jumbo transaction tests
• Existing tests pass: No regression in existing functionality
• Review: All code review feedback addressed

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🤔 Additional Information

Files to Modify

File	Changes
src/sdk/main/include/EthereumFlow.h	Update constant, add deprecation
src/sdk/tests/integration/EthereumTransactionIntegrationTests.cc	Add jumbo transaction test
src/sdk/tests/integration/EthereumFlowIntegrationTests.cc	Add jumbo transaction tests (if exists, or create)

Test Network Required

Integration tests require a running Hiero network. See existing integration tests for setup patterns, or use the CI workflow with solo-action.

References

Document	Purpose
HIP-1086	Jumbo EthereumTransaction specification

Why Deprecate EthereumFlow?

With jumbo transactions (HIP-1086):

• Cost: Direct EthereumTransaction is cheaper (no file storage fees)
• Efficiency: Single transaction vs multiple (FileCreate + FileAppend + EthereumTransaction)
• Simplicity: No file management needed

HFS fallback only needed for transactions over 128KB, which is rare.

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