Add chaos/fault-injection tests: simulated RPC failures / partially-applied storage updates

[P3az3]

Problem Statement Existing tests run in Env::default() with deterministic success paths. They do not test what happens when a host-side storage read returns a stale value, when extend_ttl is exhausted, or when a panic during mid-mutation leaves the contract in an unexpected state. Soroban's host guarantees are strong but not actually tested in this repo.

Expected Outcome A new test file chaos_tests.rs that exercises:

• Env::budget() exhaustion mid-donate (does the storage revert?).
• Forced eviction via env.storage().persistent().set of a non-existent key after a write attempt (triggers extend_ttl panic under the old code; should now no-op).
• Two parallel donate calls from the same donor in the same test (using env.ledger().with_mut_transaction or two Env instances).
• Panic during release_milestone (assert campaign state unchanged).

Acceptance Criteria

• New test file with at least 5 chaos scenarios.
• Each scenario asserts post-state correctness.
• A documented expectation table distinguishes "Soroban guarantee" vs "host implementation detail."

Implementation Notes

• This is high-value but harder to maintain — keep scope small.

Affected Files / Modules

• New: campaign/src/test/chaos_tests.rs
• campaign/src/test/mod.rs

Dependencies — None.