feat: end_campaign and permissionless update_status with auto-transition on expiry

[A6dulmalik]

Summary

Implements #35 (stacked on #55 - contains that commit since upstream main's tests currently do not compile):

• end_campaign(): creator-only early termination. Requires creator auth, rejects when frozen or when status is not Active/GoalReached, sets status to Ended, emits campaign_ended. Does NOT touch the stored campaign end time (ending is not a failure mode, so no refund window starts) and does not block the final milestone release (release_milestone validates milestone state, not campaign status).
• update_status(): permissionless transition of an expired-but-still-Active campaign to Ended, emitting campaign_ended. Anyone may call it: deadline enforcement must not depend on creator cooperation, otherwise a passive creator could keep a dead campaign nominally Active and block downstream flows keyed off status. Idempotent no-op when not expired or already ended.
• donate() now triggers the same Active -> Ended transition (with event) when a donation arrives after the deadline, so the status can never remain Active past expiry.
• Test fixes carried in this diff: added missing soroban_sdk::testutils::Ledger / IntoVal trait imports to test.rs - without these the test target fails to compile with 9 pre-existing E0599 errors (present on main and on the feat: campaign lifecycle controls (cancel, extend deadline, status) #55 branch).
• New tests: creator ends campaign + double-end rejected; expired campaign stays Active until a random account calls update_status; idempotent re-call; update_status before deadline is a no-op; post-deadline donation rejects with CampaignEnded and flips status to Ended.

Tests

Windows cannot execute Soroban host tests (environmental fail-fast), validated with:

• cargo +1.85.0 clippy -p campaign --features testutils --all-targets: compiles clean (0 errors; baseline branch had 9)
• cargo +1.85.0 fmt --check: clean

Closes #35

[ayshadogo]

LGTM