Feat/contract provider revenue withdrawal (#90)

[felixkamau]

Summary

This PR introduces a centralized treasury mechanism for the solar_grid smart contract. It replaces per-provider revenue tracking with a global treasury model and adds a configurable revenue split ratio. Tests have also been updated to align with recent Soroban SDK changes.

Related Issue

Closes #90

Type of Change

• Bug fix
• New feature
• Refactor / code cleanup
• Documentation update
• Dependency update
• Other:

Changes Made

• Configurable Treasury Share

  • Added treasury_share to initialize to define the percentage of payments allocated to the treasury.

• Centralized Revenue Tracking

  • Replaced provider_balances with a single TREASURY storage key for simplified accounting.

• Payment Logic Update

  • Updated make_payment to automatically split incoming payments based on treasury_share.

• Treasury Management Functions

  • Added:
    • withdraw_revenue for admin withdrawals
    • get_treasury to query current treasury balance
  • Removed legacy get_provider_revenue functionality.

• Test Fixes (Soroban SDK Compatibility)

  • Updated event assertions to use get_payload() to match the latest SDK behavior.

How to Test

1. Navigate to the contract directory:

   cd contracts/solar_grid
   

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[Dev-AdeTutu]

@felixkamau resolve conflict

[felixkamau]

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