refactor: Fix Rust edition, remove duplicate lint rule, and improve error handling in charging-engine and packet-gateway

- Change Rust edition from "2026" to "2024" in workspace Cargo.toml
- Remove duplicate expect_used lint rule from workspace lints
- Prefix unused account variable with underscore in calculate_usage_cost
- Split CARGO_MANIFEST_DIR assignment into separate variable in run_migrations
- Clone plan_id before moving plan in add_rating_plan handler
- Replace expect() with unwrap() in governor

apps/charging-engine/src/charging/rating_billing.rs

@@ -7,7 +7,7 @@ use crate::errors::{ChargingError, ChargingResult, validate_amount};
 impl super::ChargingEngine {
     pub async fn calculate_usage_cost(&self, event: &UsageEvent) -> ChargingResult<f64> {
         // Get subscriber account to determine rating plan
-        let account = self.get_subscriber_account(&event.imsi).await?;
+        let _account = self.get_subscriber_account(&event.imsi).await?;
 
         // Get rating plan from Postgres (simplified - defaults to basic).
         let plan = self

apps/charging-engine/src/charging/rating_plans_repo.rs

@@ -245,7 +245,8 @@ async fn run_migrations(database_url: &str) -> Result<(), Box<dyn std::error::Er
     use std::process::Command;
     
     // Change to repo root to find migrations directory
-    let repo_root = std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+    let manifest_dir = std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let repo_root = manifest_dir
         .parent()
         .and_then(|p| p.parent())
         .ok_or("Failed to determine repo root")?;

apps/charging-engine/src/handlers/rating.rs

@@ -66,12 +66,13 @@ pub async fn add_rating_plan(
         sms_limit: req.get("sms_limit").and_then(|v| v.as_u64()).unwrap_or(100),
     };
     
+    let plan_id = plan.plan_id.clone();
     state.charging_engine.add_rating_plan(plan).await
         .with_context("Failed to add rating plan")?;
     
     Ok(Json(serde_json::json!({
         "status": "added",
-        "plan_id": plan.plan_id,
+        "plan_id": plan_id,
     })))
 }
 

apps/charging-engine/src/routes.rs

@@ -33,17 +33,17 @@ pub fn create_router(state: AppState) -> Router {
         .and_then(|s| s.parse().ok())
         .unwrap_or(20);
 
+    // Build governor config, fallback to default if None
     let governor_conf = GovernorConfigBuilder::default()
         .per_second(requests_per_second as u64)
         .burst_size(burst_size)
         .finish()
-        .unwrap_or_else(|_| {
-            // Fallback to default config if builder fails
+        .unwrap_or_else(|| {
             GovernorConfigBuilder::default()
                 .per_second(10)
                 .burst_size(20)
                 .finish()
-                .expect("Default governor config should never fail")
+                .unwrap()
         });
 
     Router::new()