fix(core): resolve compilation errors and field mismatches from wave 1 contributions

[Tinna23]

Summary

This PR cleans up cascading issues introduced during wave 1 open-source contributions. All fixes are backend-only — no new features, no behaviour changes. The goal is to merge a known-good backend to main before wave 2 begins.

What was broken

routes/tx.rs

• Removed get_tx_raw handler that referenced fetch_transaction_raw — a method that was never implemented on HorizonClient
• Removed the dangling #[utoipa::path] attribute block left behind after the handler was deleted
• Removed unused imports (body::Body, response::Response, http::StatusCode, http::header) that came in with the raw handler
• Removed stale get_tx_raw entry from the OpenApi paths() macro in routes/mod.rs

routes/account.rs

• Replaced serde_json::Value return type with a typed AccountExplanationResponse struct — serde_json was not in Cargo.toml and the untyped response was causing compile failures

services/horizon.rs

• Added fetch_account() method — it existed in the route handler but was never implemented on HorizonClient, causing every account lookup to fail
• Introduced HorizonAccount intermediary struct matching Horizon's actual /accounts/:id response shape: signers as Vec<HorizonSigner>, flags as a nested HorizonAccountFlags object, home_domain as a plain String (Horizon sends "" not null)
• Added #[serde(default)] to all optional fields so unknown or absent fields do not cause deserialization failures
• Fixed into_domain() converting home_domain: String to Option<String> correctly

models/operation.rs

• Fixed From<HorizonOperation> impl: op.type_i → op.operation_type (field was renamed in HorizonOperation but the consumer was not updated)
• Fixed op.master_weight → op.master_key_weight
• Fixed op.clear_flags / op.set_flags: Horizon returns Vec<u32> but domain model expects u32 — now folded into a bitmask with .reduce(|a, b| a | b)
• Fixed op.offer_id: Horizon returns String, domain model expects u64 — now parsed with .and_then(|s| s.parse::<u64>().ok()).unwrap_or(0)
• Removed op.path reference — field does not exist on HorizonOperation

Cargo.toml

• Added serde_json = "1" to [dependencies] — was missing, causing compile failures in account route

Verified against mainnet

• Transaction lookup tested with real hashes including successful, failed, and multi-operation transactions
• Account lookup tested with real addresses — XLM balance, asset count, signer count all correct
• Timeline enrichment (ledger number + confirmed timestamp) rendering correctly

Files changed

• packages/core/src/routes/tx.rs
• packages/core/src/routes/account.rs
• packages/core/src/routes/mod.rs
• packages/core/src/services/horizon.rs
• packages/core/src/models/operation.rs
• packages/core/Cargo.toml