implemented XDR transaction builder with fee estimation

[dubemoyibe-star]

Stellar Transaction Fee Handling Improvement

Problem

The API builds unsigned Stellar XDR transactions without proper fee estimation.
This leads to transactions being rejected by the Stellar network when fees are insufficient.

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Solution

This update introduces proper fee estimation and retry handling for Stellar transactions.

Key Improvements

• Added a dedicated transaction builder service
• Integrated Horizon fee_stats for dynamic fee estimation
• Ensured all XDR builders use estimated fees
• Added support for fee bump transactions
• Implemented retry logic for fee-related failures

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New File

• src/stellar/transaction-builder.service.ts

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Updated Files

• src/stellar/stellar.service.ts
• All services responsible for building XDR transactions

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Acceptance Criteria

• Fee estimation is fetched from Horizon fee_stats
• All XDR transactions use estimated fees
• Fee bump transactions are supported
• Insufficient fee errors trigger automatic retry
• Project builds successfully with npm run build

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Mandatory Checks

• npm run build passes
• No use of any types
• Proper error handling for fee-related failures
• PR references the related issue

Closes #25

[EmeditWeb]

fix CI issues

[dubemoyibe-star]

@EmeditWeb
on it

[EmeditWeb]

fix CI issues and mc_