Reproduction steps:
1. Create a new project
2. Open Package Manager (Window > Package Management > Package Manager)
3. Press “+” button and select “Create package”
4. Enter any name and press enter to create a package
5. Select newly created package, Manage > Edit Manifest
6. In Inspector change Major, Minor and Release fields into invalid Unity versions (e.g. Major: “9999”, Minor: “9”, Release: “999b999”)
7. Press “Apply” button and observe

Actual result: Changes are saved and package manager displays Unity Version that does not exist
Expected result: Apply button is disabled and “Invalid Unity Version” error is displayed

Reproducible with: 6000.3.0b7, 6000.4.0a3
Couldn’t reproduce with: 2021.3.58f1, 2022.3.68f1, 6000.0.60f1, 6000.2.8f1 (No “Create package” option)
Reproduced on: macOS Sequoia 15.4 (M4), Windows 11