Use RESOURCE_BUSY error code for session conflicts

Changes INTERNAL_ERROR (500) to RESOURCE_BUSY (409) for "session
already exists" errors. This prevents noisy ERROR-level logs during
wrangler dev hot reloads since the SDK only logs 5xx errors.

Also improves the debug message to clarify state divergence.

Author: ghostwriternr

packages/sandbox-container/src/services/session-manager.ts

@@ -32,11 +32,10 @@ export class SessionManager {
           success: false,
           error: {
             message: `Session '${options.id}' already exists`,
-            code: ErrorCode.INTERNAL_ERROR,
+            code: ErrorCode.RESOURCE_BUSY,
             details: {
-              sessionId: options.id,
-              originalError: 'Session already exists'
-            } satisfies InternalErrorContext
+              sessionId: options.id
+            }
           }
         };
       }

packages/sandbox/src/sandbox.ts

@@ -1015,7 +1015,10 @@ export class Sandbox<Env = unknown> extends Container<Env> implements ISandbox {
     } catch (error: unknown) {
       // Session may already exist (e.g., after hot reload or concurrent request)
       if (error instanceof Error && error.message.includes('already exists')) {
-        this.logger.debug('Reusing existing session', { sessionId });
+        this.logger.debug(
+          'Session exists in container but not in DO state, syncing',
+          { sessionId }
+        );
         this.defaultSession = sessionId;
         await this.ctx.storage.put('defaultSession', sessionId);
       } else {