docs(contracts): add gas and resource fee estimation guidance for backend integrators

[ravimahatonp]

Summary

Closes #1897

Provided comprehensive gas and resource fee estimation guidance for backend engineers invoking the payment_escrow and revenue_distribution smart contracts from NestJS (backend/src/payments/soroban).

Key Changes

1. contracts/GAS_AND_FEE_ESTIMATION.md: Created a detailed fee estimation guide:
   • Breakdown of Soroban transaction costs: Inclusion Fee (base_fee) vs Resource Fee (CPU instructions, RAM memory bytes, ledger read/write entries).
   • Resource footprint benchmarks for payment_escrow methods (create, release, refund, get_status) and revenue_distribution batch distribution.
   • Recommended 20% simulation safety buffer formula to prevent BudgetExceeded panics during ledger state bumps.
   • Integration walkthrough for @stellar/stellar-sdk using simulateTransaction and handling restorePreamble for expired storage rent footprints.
   • Network congestion and failure retry policies.
2. contracts/README.md: Linked the guide under ## Gas & Fee Estimation.

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