fix: reject contract's own address as oracle in initialize

[Sundriveauto]

Closes #273

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Problem

initialize accepted any Address as the oracle without validation. Passing the contract's own address as the oracle would make oracle.require_auth() in submit_result trivially satisfiable by the contract itself, permanently compromising result submission security.

Changes

errors.rs

• Added Error::InvalidAddress = 12

lib.rs

• Changed initialize return type from () to Result<(), Error>
• Added guard: if oracle == env.current_contract_address() { return Err(Error::InvalidAddress) }
• Updated doc comment to explain the constraint and why it matters

tests.rs

• Added test_initialize_rejects_contract_address_as_oracle: registers a contract, passes its own address as oracle, asserts Error::InvalidAddress is returned

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