feat(reward-pool): add fund_pool function for USDC deposits

[Cedarich]

Description

Implements the fund_pool function to allow Protocol Admins and NGO Partners to deposit USDC into the main RewardPool contract, providing liquidity for daily tier 1 learner rewards.

Changes

• lib.rs:

  • Added PoolFunded event
  • Implemented fund_pool(env, donor, amount) function
  • Transfers USDC from donor to contract using token::Client

• test.rs:

  • Added 6 comprehensive tests covering all scenarios
  • Tests verify balance changes, event emission, and edge cases

Acceptance Criteria ✅

• ✅ Contract's USDC balance increases when funded
• ✅ Donor's USDC balance decreases accordingly
• ✅ Proper authentication required (donor.require_auth())
• ✅ PoolFunded event emitted with donor and amount

Testing

All tests passing:

• test_fund_pool_success
• test_fund_pool_emits_event
• test_fund_pool_not_initialized
• test_fund_pool_multiple_donors
• test_fund_pool_multiple_times
• test_fund_pool_zero_amount

Related Issue

User Story: Protocol Admin/NGO Partner can deposit USDC into RewardPool

closes #24

[Cedarich]

@Kaylahray please review

[Kaylahray]

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