Feature/pathfinder integration

[nicscl]

Integrating Pathfinder Route Discovery (Step 1)

This PR implements Step 1 of the Pathfinder integration: adding the core route discovery functionality. The changes include:

Changes Made

1. Added new module pathfinder.rs in crates/solver/src/interactions/ with:

   • Data structures for deserializing JSON responses
   • fetch_transfer_steps function for querying the pathfinder API
   • Comprehensive validation and error handling

2. Updated dependencies in Cargo.toml:

   • Added reqwest with JSON features
   • Added httpmock for testing

3. Added unit tests covering:

   • Successful route discovery
   • Empty route handling
   • Insufficient flow validation

Implementation Details

The implementation follows JSON-RPC standards and includes:

• Type-safe deserialization of responses
• Proper error handling and validation
• Comprehensive test coverage using mock HTTP server

Testing

All tests are passing:

running 3 tests
test interactions::pathfinder::tests::test_fetch_transfer_steps_no_steps ... ok
test interactions::pathfinder::tests::test_fetch_transfer_steps_insufficient_flow ... ok
test interactions::pathfinder::tests::test_fetch_transfer_steps_success ... ok

Next Steps

This PR sets up the foundation for:

1. Integration with circle_ubi.rs for settlement transaction generation
2. Handling scenarios where no route is available
3. Adding configuration for pathfinder endpoint URLs

Notes

• The endpoint URL will need to be configured based on the deployment environment
• Error handling follows the codebase's conventions using anyhow
• All new code is documented with rustdoc comments

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