Add Test: oracle duplicate submit_result returns AlreadySubmitted error code

**Priority:** Medium
**Estimated Time:** 15 minutes

**Description:**
`test_duplicate_submit_fails` uses `#[should_panic]` without asserting the specific error code. A `try_submit_result` variant should assert `Error::AlreadySubmitted` explicitly.

**Tasks:**
- Call `try_submit_result` twice for the same `match_id`
- Assert the second call returns `Err(Ok(Error::AlreadySubmitted))`