chore: fix JS SDK default import for @stellar/stellar-sdk v16.0.0 (#2521)

js-stellar-sdk v16.0.0 removed the ESM default export (and merged
@stellar/stellar-base into @stellar/stellar-sdk). The string-to-ScVal
example used `import StellarSdk from "@stellar/stellar-sdk"`, which no
longer works. Switch to a namespace import (`import * as StellarSdk`)
to preserve the `StellarSdk.xdr.ScVal` body unchanged.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ElliotFriend

docs/build/guides/conversions/string-conversions.mdx

@@ -90,7 +90,7 @@ pub fn string_to_val(string: String) -> Val {
 ```
 
 ```js
-import StellarSdk from "@stellar/stellar-sdk";
+import * as StellarSdk from "@stellar/stellar-sdk";
 
 // Example string value
 const stringValue = "Hello, Stellar!";