feat: add get rewards balance view

[somotochukwu-dev]

Add get_rewards_balance view function to rewards contract

Adds a get_rewards_balance(owner: Address) -> i128 view function that returns the current unclaimed reward balance for an address.

Changes

contracts/rewards/src/rewards.rs — Added get_rewards_balance module-level function that delegates to the existing get_reward_balance storage helper. Returns 0 when no account or zero balance.

contracts/rewards/src/lib.rs — Added import and get_rewards_balance entry point on RewardsContract.

Tests added

• test_get_rewards_balance_returns_balance_after_credit — returns correct balance after crediting rewards.
• test_get_rewards_balance_returns_zero_for_unregistered — returns 0 for an address with no account.
• test_get_rewards_balance_returns_zero_for_fresh_account — returns 0 for a newly registered account.
• test_get_rewards_balance_reflects_debits — balance correctly reflects debits.

Closes #974

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[Uyoxy]

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