Fix stale WRENN_SANDBOX_ID and WRENN_TEMPLATE_ID after snapshot restore

After restoring a VM from snapshot, envd had already completed its initial
MMDS poll, so the metadata files in /run/wrenn/ and env vars retained values
from the original sandbox. Call POST /init after WaitUntilReady on both
resume and create-from-template paths to trigger envd to re-read MMDS.

internal/envdclient/client.go

@@ -268,6 +268,30 @@ func (c *Client) ReadFile(ctx context.Context, path string) ([]byte, error) {
 	return data, nil
 }
 
+// PostInit calls envd's POST /init endpoint, which triggers a re-read of
+// Firecracker MMDS metadata. This updates WRENN_SANDBOX_ID, WRENN_TEMPLATE_ID
+// env vars and the corresponding files under /run/wrenn/ inside the guest.
+// Must be called after snapshot restore so envd picks up the new sandbox's metadata.
+func (c *Client) PostInit(ctx context.Context) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.base+"/init", nil)
+	if err != nil {
+		return fmt.Errorf("create request: %w", err)
+	}
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("post init: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusNoContent {
+		body, _ := io.ReadAll(resp.Body)
+		return fmt.Errorf("post init: status %d: %s", resp.StatusCode, string(body))
+	}
+
+	return nil
+}
+
 // ListDir lists directory contents inside the sandbox.
 func (c *Client) ListDir(ctx context.Context, path string, depth uint32) (*envdpb.ListDirResponse, error) {
 	req := connect.NewRequest(&envdpb.ListDirRequest{