feat(core): add GET /tx/:hash/raw endpoint returning unprocessed Horizon response

[demilade18-git]

Summary

• Adds GET /tx/:hash/raw route that proxies the raw Horizon JSON body directly to the caller with Content-Type: application/json
• Adds fetch_transaction_raw to HorizonClient using res.bytes() to avoid deserialisation
• Applies identical error handling to /tx/:hash — 400 for invalid hash, 404 for not found, 502 for upstream failures
• Registers the route in the router and OpenAPI spec
• Documents the endpoint in the README
• Wires up horizon_test.rs into the module tree (was previously unreachable) and adds 3 tests covering the happy path, 404, and 5xx cases

Test plan

• cargo test passes (170 tests, 0 failures)
• GET /tx/:hash/raw returns the full Horizon JSON body unmodified
• GET /tx/:hash/raw with an invalid hash returns 400
• GET /tx/:hash/raw with an unknown hash returns 404

Closes #85