Add max_supply cap enforcement to claim_reward function

[DeFiVC]

What

The claim_reward function in learn-token mints tokens by directly writing to balance and supply storage, bypassing the max_supply check that mint() enforces.

Why

If max_supply is set to a value lower than the total rewards that would be minted over time, the claim_reward path will exceed the cap. This undermines the supply cap guarantee — the primary token distribution path doesn't respect it.

Scope

• Add the same max_supply check from mint() to claim_reward
• Ensure the check uses the same logic and error message
• Add a test verifying the cap is enforced

Technical Context

• File: contracts/learn-token/src/lib.rs (lines ~418-423 for claim_reward, ~359-363 for mint)
• claim_reward writes directly to storage: storage::write_balance and storage::write_supply
• mint checks: if current_supply + amount > max_supply { panic!("maximum supply cap exceeded"); }

Acceptance Criteria

• claim_reward panics when minting would exceed max_supply
• The check is identical to the one in mint()
• Unit test verifies the cap is enforced in the reward path

[addnad]

i’ve gone through the description expect a pr in less than 12hrs