Custom Package incorrect name error is not informative when the name has an uppercase letter

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Open the Package Manager window (Window > Package Management > Package Manager)
3. Create a custom package named "asd" by pressing the "+" button in the Package Manager window
4. Select the "asd" package in the Package Manager
5. Click the "Edit" button in the Inspector window
6. Change the "Name" field to "Asd"
7. Observe the error in the Inspector window

Actual results: Invalid Package Name 'asd'

Expected results: The error should tell the user specifically what is wrong with the name, for example that the user cannot use uppercase letters

Reproducible with versions: 6000.3.13f1, 6000.4.3f1, 6000.5.0b4, 6000.6.0a2

Can’t test with versions: 6000.0.73f1 (due to no custom package functionality)

Tested on (OS): MacOS Silicon Tahoe 26.3, Ubuntu 24.04