feat(api)!: fix $getActiveIdentity return type for ActiveDid migration

Update $getActiveIdentity() method to return { activeDid: string } instead
of full ActiveIdentity object. Add validation to ensure activeDid exists
in accounts table and clear corrupted values. Update migration plan to
reflect completed first step of API layer implementation.

- Change return type from Promise<ActiveIdentity> to Promise<{ activeDid: string }>
- Add account validation with automatic corruption cleanup
- Simplify query to only select activeDid field
- Improve error handling to return empty string instead of throwing
- Update migration plan documentation with current status

src/utils/PlatformServiceMixin.ts

@@ -553,33 +553,41 @@ export const PlatformServiceMixin = {
      * Get active identity from the new active_identity table
      * This replaces the activeDid field in settings for better architecture
      */
-    async $getActiveIdentity(): Promise<ActiveIdentity> {
+    async $getActiveIdentity(): Promise<{ activeDid: string }> {
       try {
         const result = await this.$dbQuery(
-          "SELECT id, activeDid, lastUpdated FROM active_identity WHERE id = 1",
+          "SELECT activeDid FROM active_identity WHERE id = 1",
         );
 
         if (result?.values?.length) {
-          const [id, activeDid, lastUpdated] = result.values[0];
-          return {
-            id: id as number,
-            activeDid: activeDid as string,
-            lastUpdated: lastUpdated as string,
-          };
+          const activeDid = result.values[0][0] as string;
+          
+          // Validate activeDid exists in accounts
+          if (activeDid) {
+            const accountExists = await this.$dbQuery(
+              "SELECT did FROM accounts WHERE did = ?",
+              [activeDid]
+            );
+            
+            if (accountExists?.values?.length) {
+              return { activeDid };
+            } else {
+              // Clear corrupted activeDid
+              await this.$dbExec(
+                "UPDATE active_identity SET activeDid = '', lastUpdated = datetime('now') WHERE id = 1"
+              );
+              return { activeDid: "" };
+            }
+          }
         }
-
-        // Return default if no record exists
-        return {
-          id: 1,
-          activeDid: "",
-          lastUpdated: new Date().toISOString(),
-        };
+        
+        return { activeDid: "" };
       } catch (error) {
         logger.error(
           "[PlatformServiceMixin] Error getting active identity:",
           error,
         );
-        throw error;
+        return { activeDid: "" };
       }
     },