fix: enforce campaign end_time in donate() (closes #5)

[Carlys17]

Summary

Fixes the security gap where donate() did not enforce campaign.end_time. Donations could continue past the deadline until the creator manually called end_campaign().

Closes #5

Changes

campaign/src/lib.rs

• Added deadline gate inside the Active | GoalReached match arm in donate():

  let now = env.ledger().timestamp();
  match campaign.status {
      CampaignStatus::Active | CampaignStatus::GoalReached => {
          if now >= campaign.end_time {
              panic_with_error!(env, Error::CampaignEnded);
          }
      }
      _ => panic_with_error!(env, Error::CampaignNotActive),
  }

• Panics with Error::CampaignEnded = 4 (already defined, previously unused in production code)
• Check positioned after is_frozen but before min_donation_amount — no state mutation on a doomed call
• Reentrancy lock acquired before the rejection check (no early-return on a held lock)
• Doc-comment on donate updated to mention the deadline gate

campaign/src/test/negative_path_tests.rs (+4 tests)

• test_donate_fails_past_deadline_active — Active + past deadline → reject
• test_donate_fails_past_deadline_goal_reached — GoalReached + past deadline → reject
• test_donate_fails_at_exact_deadline_boundary — boundary: now == end_time → reject (strict >=)
• test_donate_succeeds_just_before_deadline — now == end_time - 1 → succeeds

campaign/src/test/integration_tests.rs (+1 test)

• test_lifecycle_donate_before_deadline_succeeds — positive lifecycle: donate before deadline accepted

campaign/src/test/invariant_tests.rs (+2 tests)

• invariant_donate_before_deadline_always_succeeds — positive companion: Active & GoalReached before deadline → succeeds
• invariant_donate_at_or_past_deadline_always_fails — negative companion: donate(clk >= end_time).is_err() (should_panic with Error::CampaignEnded)

docs/events.md

• Documented the deadline gate on donation_received event section

Acceptance Criteria Checklist

• New typed error returned (Error::CampaignEnded = 4) when now >= end_time while status is non-terminal
• Existing tests pass unchanged
• Negative-path tests: Active past deadline, GoalReached past deadline, exact boundary
• docs/events.md and doc-comment on donate updated
• Reentrancy lock acquired before the rejection check

Implementation Notes

• Auto-transitioning to Ended status on the first late call was considered but deemed out-of-scope for this security fix. The current approach (reject + panic) is the minimal safe change. A follow-up issue could add auto-transition if desired.
• Error::CampaignEnded = 4 was already defined in the Error enum but unused in any production code path — this PR puts it to its intended use.

Test Results

All 158 tests pass. Clippy clean. Fmt clean.

[P3az3]

@Carlys17 please remove test snapshots

[P3az3]

@Carlys17 resolve conflicts

[P3az3]

LGTM