Implement cross-chain payment routing helpers with bridge fee estimation

[Kingsman-99]

Overview

Add SDK helpers for routing payments from non-Stellar chains (Ethereum, Solana) toward a StellarSplit invoice via a bridge. The SDK must estimate bridge fees, build the relay proof parameters, and submit the bridge payment to the contract.

Requirements

• client.estimateBridgeFee(sourceChain: ChainId, amount: bigint): Promise<BridgeFeeEstimate>
• BridgeFeeEstimate { bridgeFee: bigint, netAmount: bigint, estimatedTimeSeconds: number }
• client.buildBridgePayment(params: BridgePaymentParams): BridgePaymentRequest — builds the unsigned relay proof struct
• client.submitBridgePayment(proof: SignedBridgeProof): Promise<{ txHash: string }> — submits to contract
• Support at minimum: Ethereum mainnet and Solana mainnet as source chains
• Write tests with mock bridge fee APIs and mock contract responses

Acceptance Criteria

• Fee estimation returns valid estimates for both chains
• Bridge payment request builds correctly
• Submission calls the contract's bridge_pay entry point
• npm test passes with zero failures
• TypeScript compiles with zero errors
• ESLint passes with zero errors

Definition of Done

All CI checks must pass before the PR is reviewed.

[nurudeenheroic]

Hello! I've reviewed the issue and can implement an efficient fix.

[Tobi-8]

Hey maintainer, I have experience in handling this issue
Kindly assign

[Davichi-1]

Hello Maintainer, I'd like to work on this issue. I have experience with frontend development and state management, and I can implement this. Please consider assigning it to me.

[obedebuka41-dotcom]

i will like to fix this

[grantfox-oss]

🦊 GrantFox — @obedebuka41-dotcom has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #426)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.