fix(core): Sanitize lone surrogates in parameterized (fmt) log bodies

`parameterize`/`fmt` creates a `String` object via `new String()`, so
`typeof message` returns `'object'` not `'string'`, bypassing the
sanitization. Use `String(message)` to coerce to a primitive before
sanitizing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antonis

packages/core/src/logs/internal.ts

@@ -162,7 +162,7 @@ export function _INTERNAL_captureLog(
   const serializedLog: SerializedLog = {
     timestamp,
     level,
-    body: typeof message === 'string' ? _INTERNAL_removeLoneSurrogates(message) : message,
+    body: _INTERNAL_removeLoneSurrogates(String(message)),
     trace_id: traceContext?.trace_id,
     severity_number: severityNumber ?? SEVERITY_TEXT_TO_SEVERITY_NUMBER[level],
     attributes: sanitizeLogAttributes({

packages/core/test/lib/logs/internal.test.ts

@@ -1280,6 +1280,19 @@ describe('_INTERNAL_captureLog', () => {
       expect(logBuffer?.[0]?.body).toBe('bad surrogate \uFFFD here');
     });
 
+    it('sanitizes lone surrogates in parameterized (fmt) log message body', () => {
+      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, enableLogs: true });
+      const client = new TestClient(options);
+      const scope = new Scope();
+      scope.setClient(client);
+
+      const badValue = 'bad\uD800value';
+      _INTERNAL_captureLog({ level: 'error', message: fmt`parameterized ${badValue} message` }, scope);
+
+      const logBuffer = _INTERNAL_getLogBuffer(client);
+      expect(logBuffer?.[0]?.body).toBe('parameterized bad\uFFFDvalue message');
+    });
+
     it('sanitizes lone surrogates in log attribute values', () => {
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, enableLogs: true });
       const client = new TestClient(options);