Refactor file path concatenation to use Path.Combine

[FireRat666]

The SDK had hardcoded Windows backslashes:
var file = Path.Join(assetBundleRoot, assetBundleDirectory) + "\\" + name;

On Linux:
The path became "Assets/WebRoot\windows.banter".
Because Linux file systems do not treat \ as a directory separator, File.Exists("Assets/WebRoot\windows.banter") returned false.
The SDK logged: File not found, skipping: Assets/WebRoot\windows.banter and exited the upload routine without uploading the files.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 7255e74a-5784-413d-8b52-1b3fa7dc82a5

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

_{Comment @coderabbitai help to get the list of available commands.}