Add sovereign cloud support to AzureCloudInstance enum

[Copilot]

Changes proposed in this request

PR #5671 added instance discovery support for Bleu (France), Delos (Germany), and GovSG (Singapore) sovereign clouds, but the AzureCloudInstance enum was not updated to expose them.

This PR adds:

• Three new enum values: AzureBleu, AzureDelos, AzureGovSG
• URL mappings in AuthorityInfo.GetCloudUrl() for the new clouds
• Test coverage for all authority audience combinations

Developers can now use the enum instead of hardcoding URLs:

// Before: had to use custom authority URL
var app = PublicClientApplicationBuilder.Create(clientId)
    .WithAuthority("https://login.sovcloud-identity.fr/tenantId/")
    .Build();

// After: use the enum
var app = PublicClientApplicationBuilder.Create(clientId)
    .WithAuthority(AzureCloudInstance.AzureBleu, tenantId)
    .Build();

Testing

Added 9 test cases covering all three new clouds with AzureAdMultipleOrgs, AzureAdAndPersonalMicrosoftAccount, and PersonalMicrosoftAccount audiences.

Performance impact

None. Changes are additive enum values and switch cases.

Documentation

• All relevant documentation is updated.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Documentation] Sovereign cloud usage examples and AzureCloudInstance enum</issue_title>
<issue_description>Documentation related to component
Authentication to sovereign clouds

Please check those that apply

• typo
• documentation doesn't exist
• documentation needs clarification
• error(s) in example
• needs example

Description of the issue
In PR #5671, support for sovereign clouds (Bleu, Delos, GovSG) has been added to MSAL.Net.

It would be great to have a usage example on how to authenticate against these clouds.

I would also like to see how the Microsoft.Identity.Client.AzureCloudInstance enum is connected to this, as the enum does not yet contain any hint to the new sovereig clouds.
</issue_description>

<agent_instructions>Update the AzureCloudInstance enum</agent_instructions>

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

• Fixes [Documentation] Sovereign cloud usage examples and AzureCloudInstance enum #5706

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