Integrate on-chain price oracle for dynamic invoice funding targets

[Kingsman-99]

Overview

Add oracle-priced invoices where the funding target is determined at payment time by an external price oracle contract, not fixed at creation. Use case: invoice 100 USD worth of XLM where the XLM/USD rate is fetched from a Stellar oracle at payment time.

Requirements

• Extend InvoiceOptions with oracle: Option<Address> and oracle_asset_pair: Option<(Symbol, Symbol)>
• At payment time, if oracle is set, call the oracle contract's price(asset_pair) function via env.invoke_contract and compute the required token amount dynamically
• Store the oracle address as part of invoice state
• Emit OraclePriceFetched { invoice_id, rate, computed_amount } event on each payment
• If the oracle call fails (stale/unavailable), panic with OracleUnavailable
• Write tests using a mock oracle contract that returns configurable rates
• Test: oracle price changes between payments; oracle unavailable; oracle returns zero

Acceptance Criteria

• Oracle integration working end-to-end in tests
• Mock oracle contract in tests/ used for all test scenarios
• Oracle failure panics with OracleUnavailable, not a cryptic error
• All existing tests pass
• cargo test --workspace passes with zero failures
• cargo clippy --all-targets -- -D warnings produces zero warnings

Definition of Done

All CI checks must pass before the PR is reviewed.

[zeemscript]

Hello @maintainer, I’ve gone through the requirements and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me.

[lemarjohnny781]

please i would like to work on this issue

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #347)
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