"Save As" field is empty when creating a new StyleSheet from the Style Sheets or the UI Builder windows

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Open the UI Builder (Window > UI Toolkit > UI Builder)
3. Click the "+" button in the StyleSheets tab and select "Create New USS"
4. Observe the "Save USS File" popup

Actual results: The "Save As" field is empty

Expected results: The "Save As" field is pre-filled with a default name ("New USS")

Reproducible with versions: 6000.0.0f1, 6000.0.82f1, 6000.3.23f1, 6000.5.11f1, 6000.6.0f1, 6000.7.0a6

Tested on (OS): MacOS Silicon Tahoe 26.6.2

Notes:

• This is a design inconsistency as the UI Document has the "Save As" field pre-filled, as well as creating a new StyleSheet in the Project window - so the same asset gets a default name from one entry point but not the other
• Reproduces in both UITK editing stage (in-scene) and the UI Builder