feat: add oracle price feed verification integration test

[sanmipaul]

Summary

`contracts/tests/integration_tests.rs` has no test that verifies oracle price feed integration. The pool uses collateral valuations that depend on oracle prices, but no test covers what happens when:

1. The oracle returns a stale price
2. The oracle returns zero (divide-by-zero risk)
3. The oracle price changes between collateral deposit and seizure check
4. The oracle is unavailable (cross-contract call fails)

Missing Test Cases

• `test_stale_oracle_price_rejected`: pool should reject collateral valuations from stale oracle feeds
• `test_oracle_zero_price_handled`: oracle returning 0 should not cause divide-by-zero
• `test_price_change_between_deposit_and_seizure`: collateral value at seizure time reflects current oracle price
• `test_oracle_unavailable_fallback`: if oracle call fails, pool should return a safe error

Implementation Notes

• Use a mock oracle contract in the Soroban test environment
• The mock should be configurable to return specific prices, timestamps, and errors
• Tests should set up the mock, perform operations, then verify behavior

Acceptance Criteria

• Mock oracle contract created for test use
• All 4 test cases above implemented and passing
• Tests run in CI without external oracle dependency
• Each test asserts the specific expected error or behavior

References

• `contracts/tests/integration_tests.rs`
• `contracts/pool/src/lib.rs` — oracle price usage in collateral valuation

[Amas-01]

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[Obiajulu-gif]

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