feat: add reward pool balance validation before hunt activation

[otobongdev]

Description

Adds reward pool balance validation before hunt activation. When a reward manager is configured, activate_hunt now verifies that the pool balance is sufficient to cover min_distribution_amount * max_winners before allowing the hunt to be activated.

Acceptance Criteria

• Check pool balance >= min_distribution_amount * max_winners
• Return descriptive error (InsufficientRewardPool) if insufficient
• Optional: warn but allow activation with flag (not implemented - can be added in follow-up)

Changes

contracts/reward-manager/src/lib.rs

• Added get_min_distribution_amount public getter that returns the min_distribution_amount from the pool config (or 0 if no pool exists)

contracts/hunty-core/src/lib.rs

• Updated activate_hunt to query the reward manager for min_distribution_amount via cross-contract call
• Added validation: pool_balance >= min_distribution_amount * max_winners (using saturating_mul for overflow safety)
• Returns HuntErrorCode::InsufficientRewardPool with detailed error info when validation fails
• Validation only enforced when min_distribution_amount > 0 (0 = no minimum constraint)

Testing

• cargo build -p reward-manager && cargo build -p hunty-core
• cargo test -p hunty-core && cargo test -p reward-manager

Closes #360