Log malformed message contents in base64 (#272)

## Why

Noticed a number of malformed message contents in DD
https://us5.datadoghq.com/logs?query=%22received%20malformed%20msg%22%20%40environment%3Aproduction&agg_m=count&agg_m_source=base&agg_t=count&cols=host%2Cservice%2C%40riverTarget&fromUser=true&messageDisplay=inline&refresh_mode=sliding&storage=hot&stream_sort=desc&viz=stream&from_ts=1726796110823&to_ts=1727400910823&live=true

However we are sending binary messages originally, so we can't tell what
the original contents were once its decoded to a string. We can base64
encode the buffer and then reverse it when trying to debug what is going
wrong with these messages

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## What changed

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the reviewer -->

## Versioning

- [ ] Breaking protocol change
- [ ] Breaking ts/js API change

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transport/sessionStateMachine/common.ts

@@ -160,9 +160,8 @@ export abstract class CommonSession extends StateMachineState {
     const parsedMsg = this.options.codec.fromBuffer(msg);
 
     if (parsedMsg === null) {
-      const decodedBuffer = new TextDecoder().decode(Buffer.from(msg));
       this.log?.error(
-        `received malformed msg: ${decodedBuffer}`,
+        `received malformed msg: ${Buffer.from(msg).toString('base64')}`,
         this.loggingMetadata,
       );