Fix Identity Name validation to enforce MSIX schema pattern

[Copilot]

The CleanPackageName method was allowing underscores and spaces in Identity Names, violating the MSIX schema pattern [-.A-Za-z0-9]+ and causing create-debug-identity failures.

Changes

• Updated regex pattern from [^A-Za-z0-9\-_. ] to [^A-Za-z0-9.\-] to reject underscores and spaces
• Removed obsolete logic for prepending "App" to names starting with underscore (net -8 lines)
• Added test coverage for underscore/space removal and mixed character handling

Behavior

CleanPackageName("My_Test_Package")          // => "MyTestPackage"
CleanPackageName("Test Package Name")        // => "TestPackageName"  
CleanPackageName("Valid.Package-Name.123")   // => "Valid.Package-Name.123" (unchanged)

Identity Names now comply with MSIX AppxManifest schema requirements.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: A new manifest generates a non valid Identity name</issue_title>
<issue_description>### Describe the bug

When creating a new manifest (tested via winapp init), if the provided name contains underscore, it will happily use that name for the Identity Name even though it is not valid per the schema.

To Reproduce

1. run winapp init
2. Set the name to something that contains undescore (_)
3. Try to create debug identity with winapp create-debug-identity
4. Error

Expected behavior

The Name property of the Identity object should be valid per the schema [-.A-Za-z0-9]+

Screenshots

No response

OS Version and details

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: A new manifest generates a non valid Identity name #258

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