test(events): bounty pillar — apply / withdraw_application + credit charge

[0xdevcollins]

Part of #25 (contract test-coverage push).

events

How to test

• Reuse the crate's tests/common.rs harness; add tests/bounty_pillar.rs + a one-line mod bounty_pillar; in tests/mod.rs.
• Template: contracts/events/src/tests/cross_contract.rs.
• Run cargo test -p boundless-events, then cargo test --all before the PR.

Acceptance criteria

• New tests/bounty_pillar.rs covering every function in scope.
• Happy path + each Error variant + edge cases + auth-rejection (+ idempotency where relevant).
• cargo test --all passes; PR links this issue.

[dev8967]

Implemented in PR #43: #43