feat: implement integration test suite and CI entrypoint for full flows

[greatest0fallt1me]

Description:

Add a dedicated integration test suite that runs full flows (create event → place bets → resolve → claim) with mock oracles and tokens, and document CI entrypoint (e.g., cargo test --test integration).

Requirements and Context:

• Must be secure, tested, and documented
• Should add integration tests (e.g., in tests/ or src/integration_test.rs) covering: create event, place bets, resolve (oracle/manual), claim, cancel, refund
• Should use Soroban testutils and mock contracts where needed
• Should document how to run: cargo test --test integration or similar
• Should be runnable in CI (no network required)
• Must not replace unit tests; complement them

Suggested Execution:

1. Fork the repo and create a branch

   git checkout -b feature/integration-test-suite

2. Implement changes

   • Add integration test file(s)
   • Add README or Makefile target for integration tests
   • Add documentation

3. Test and commit

   • Run full test suite and integration tests; include test output

Example Commit Message:

feat: implement integration test suite and CI entrypoint for full flows

Guidelines:

• Clear documentation
• Timeframe: 72 hours

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