looprpc: populate static address swap timing and cost fields

[hieblmi]

• Added initiation_time, last_update_time, cost_server, cost_onchain, and cost_offchain to StaticAddressLoopInSwap.
• Populated static loop-in list responses with stored initiation/update timestamps.
• Reported the persisted quoted swap fee as cost_server once the static loop-in invoice has been paid.
• Left on-chain/off-chain costs at zero because static timeout-path miner fees are not currently persisted.
• Added focused test coverage for static loop-in cost reporting behavior.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the static address loop-in functionality by exposing timing and cost information through the RPC interface. By updating the protobuf definitions and implementing logic to calculate and report these metrics, users can now gain better visibility into the lifecycle and financial performance of their static address swaps.

Highlights

• API Schema Updates: Extended the StaticAddressLoopInSwap RPC message to include initiation_time, last_update_time, cost_server, cost_onchain, and cost_offchain fields.
• Cost Reporting Logic: Implemented staticAddressLoopInSwapCost to populate swap costs, ensuring server costs are reported once the invoice is paid, while keeping on-chain and off-chain costs at zero as they are not currently persisted.
• Testing: Added comprehensive test coverage in TestStaticAddressLoopInSwapCost to verify the correct reporting of swap costs across different swap states.

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[gemini-code-assist]

Code Review

This pull request adds initiation time, last update time, and cost fields (server, on-chain, off-chain) to the StaticAddressLoopInSwap protobuf message and populates them in the server implementation. It also introduces a helper function to calculate these costs and adds corresponding unit tests. The review feedback correctly identifies a potential issue where calling UnixNano() on uninitialized time.Time fields can lead to unexpected negative timestamps, and suggests adding safety checks for zero-time values and nil pointers.