Add Name parameter to UserPasskeyInfo constructor

[BloodShop]

Summary

UserPasskeyInfo exposes a Name property but the constructor doesn't accept it — it's the only property left out. Callers have to fall back to an object initializer:

var passkey = new UserPasskeyInfo(credentialId, publicKey, ...)
{
    Name = "My Security Key"  // why can't this go in the constructor?
};

This adds a constructor overload so Name can be set inline with everything else.

What changed

UserPasskeyInfo gets a second constructor with an additional trailing string? name parameter. The original constructor delegates to it with name: null, so nothing breaks — existing call sites compile and behave exactly as before.

UserPasskeyInfo.cs

• New constructor overload: ..., byte[] clientDataJson, string? name)
• Original constructor chains into it via : this(..., name: null)

PublicAPI.Unshipped.txt

• New API entry for the overloaded constructor

UserPasskeyInfoTest.cs (new file)

• Verifies the new constructor sets all properties including Name
• Verifies passing null leaves Name unset
• Verifies the original constructor still works and leaves Name null

Usage

// Before (still works)
var passkey = new UserPasskeyInfo(credentialId, publicKey, createdAt,
    signCount, transports, isUserVerified, isBackupEligible, isBackedUp,
    attestationObject, clientDataJson)
{
    Name = "My Security Key"
};

// After (also works now)
var passkey = new UserPasskeyInfo(credentialId, publicKey, createdAt,
    signCount, transports, isUserVerified, isBackupEligible, isBackedUp,
    attestationObject, clientDataJson, name: "My Security Key");

Breaking changes

None. The existing constructor signature is untouched and remains in PublicAPI.Shipped.txt. The new overload is additive only.

Fixes #65774

[dotnet-policy-service]

Thanks for your PR, @@BloodShop. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[BloodShop]

@dotnet-policy-service agree