fix the claim refund donors

[KAMALDEEN333]

claim_refund` does not refund donors whose only donations were Native XLM
closes #11

Description
Problem Statement

In CampaignContract::claim_refund (campaign/src/lib.rs), the per-asset refund loop is:

for asset in campaign.accepted_assets.iter() {
let asset_address = match &asset.issuer {
Some(addr) => addr.clone(),
None => continue, // ⚠️ Native XLM donors silently skipped
};
...
}
The continue; drops any donor whose donations were entirely in Native XLM. Combined with Issue 1 (donors past the deadline can still donate if status is non-terminal) and Issue 2 (Native routes through accepted_assets[].issuer), there is a path where a Native-only donor believes they are entitled to a refund but receives nothing.

Why it matters

This is a direct failure of the documented refund contract. RefundNotPermitted is only returned when none of the eligibility checks pass. A Native-only donor whose refund is otherwise eligible will lose funds.

Expected Outcome

The per-asset refund loop must materialize the network's canonical wrapped-XLM contract address (introduced in Issue 2) and use it as the asset_address for any native donations. For multi-asset campaigns, each accepted asset gets a corresponding refund path.

Acceptance Criteria

New test test_claim_refund_native_xlm_donor that donates Native, cancels the campaign, and asserts successful refund with the canonical XLM contract address.
New test test_claim_refund_mixed_native_and_sac that donates both Native and a custom SAC, cancels the campaign, and asserts successful refunds for both.
Negative tests:
Campaign with no native XLM entry in accepted_assets rejects Native donations (the donation-side fix from Issue 2 complements this).
docs/events.md updates the refund_claimed event example with a Native contribution.
Implementation Notes

Pull out a fn xlm_token_address(env: &Env) -> Address returning the network-fixed canonical wrapped XLM. Use it in both donate and claim_refund.
Replace the inner continue with the same loop body but parameterized with the canonical XLM address for None issuer entries.
Affected Files / Modules

campaign/src/lib.rs (claim_refund)
campaign/src/types.rs (StellarAsset, xlm constants)
campaign/src/test/claim_refund_tests.rs (new tests)
campaign/src/test/refund_eligibility_tests.rs (mixed-asset test)
Dependencies — Depends on Issue 2 (canonical native XLM address).

[P3az3]

LGTM