refactor: BatteryInfo structure (consistent layout)

- use tagged enum serialization for BatteryError
- flatten BatteryInfo telemetry with consistent field names and typed error

rust/src/main.rs

@@ -43,7 +43,7 @@ use embassy_time::{Duration, Instant, Timer};
 use esp_hal::rom::ets_delay_us;
 use esp_hal::system::software_reset;
 use esp_println::{logger, println};
-use hal::battery::BatteryState;
+use hal::battery::{BatteryError, BatteryInfo, BatteryState};
 use log::LogMessage;
 use option_lock::OptionLock;
 use plant_state::PlantState;
@@ -790,20 +790,40 @@ async fn publish_mppt_state(
 async fn publish_battery_state(
     board: &mut MutexGuard<'_, CriticalSectionRawMutex, HAL<'static>>,
 ) -> () {
-    let state = board
+    let telemetry = match board
         .board_hal
         .get_battery_monitor()
         .get_battery_state()
-        .await;
-    let value = match state {
-        Ok(state) => {
-            let json = serde_json::to_string(&state).unwrap().to_owned();
-            json.to_owned()
-        }
-        Err(_) => "error".to_owned(),
-    };
+        .await
     {
-        let _ = mqtt::publish("/battery", &*value).await;
+        Ok(BatteryState::Info(info)) => info,
+        Ok(BatteryState::Unknown) => BatteryInfo {
+            voltage_mv: None,
+            avg_current_ma: None,
+            soc_pct: None,
+            soh_pct: None,
+            temperature_c: None,
+            cycle_count: None,
+            remaining_mah: None,
+            design_mah: None,
+            error: Some(BatteryError::NoBatteryMonitor),
+        },
+        Err(e) => BatteryInfo {
+            voltage_mv: None,
+            avg_current_ma: None,
+            soc_pct: None,
+            soh_pct: None,
+            temperature_c: None,
+            cycle_count: None,
+            remaining_mah: None,
+            design_mah: None,
+            error: Some(BatteryError::CommunicationError {
+                message: alloc::format!("{:?}", e),
+            }),
+        },
+    };
+    if let Ok(json) = serde_json::to_string(&telemetry) {
+        let _ = mqtt::publish("/battery", &json).await;
     }
 }