Document breaking change: DefaultAzureCredential defaults to ManagedIdentityCredential on ACA and App Service

[Copilot]

This PR documents a breaking change in Aspire 13.0 where DefaultAzureCredential behavior changes when deploying to Azure Container Apps and Azure App Service.

Background

With dotnet/aspire#11832, Aspire now automatically sets the AZURE_TOKEN_CREDENTIALS environment variable when deploying to Azure Container Apps and Azure App Service. This forces DefaultAzureCredential to only use ManagedIdentityCredential instead of trying the full chain of credential providers (EnvironmentCredential, WorkloadIdentityCredential, then ManagedIdentityCredential).

Changes Made

This PR adds comprehensive documentation for this breaking change:

• Created docs/compatibility/13.0/ folder structure - New directory for Aspire 13.0 breaking changes
• Added breaking change document - defaultazurecredential-managedidentity-default.md with:
  • Clear explanation of previous vs. new behavior
  • Rationale based on Azure SDK best practices for deterministic credentials
  • Two recommended workaround options with code examples:
    1. Explicitly use EnvironmentCredential or WorkloadIdentityCredential instead of DefaultAzureCredential
    2. Remove the AZURE_TOKEN_CREDENTIALS environment variable via PublishAsAzureContainerApp or PublishAsAzureAppService callbacks
  • Proper xref links for affected APIs (AddAzureContainerAppEnvironment, AddAzureAppServiceEnvironment)
• Created index page - docs/compatibility/13.0/index.md for the 13.0 breaking changes
• Updated TOC - Added Aspire 13.0 section to docs/compatibility/toc.yml

Documentation Quality

• ✅ Followed all guidelines from .github/prompts/breaking-change.md
• ✅ Markdown linting passed
• ✅ Code review completed with no issues
• ✅ Includes ai-usage: ai-assisted frontmatter as required
• ✅ Past tense for previous behavior, present tense for new behavior
• ✅ Links to Azure SDK best practices and related PRs

Fixes #5154

Original prompt

Please create a draft PR to address @dotnet/docs-aspire/issues/5154 - also please rely on the @dotnet/docs-aspire/files/.github/prompts/breaking-change.md prompt to help guide you through the creation of this PR.

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Internal previews

📄 File	🔗 Preview link
docs/compatibility/13.0/defaultazurecredential-managedidentity-default.md	DefaultAzureCredential defaults to ManagedIdentityCredential on ACA and App Service
docs/compatibility/13.0/index.md	Breaking changes in Aspire 13.0
docs/compatibility/toc.yml	docs/compatibility/toc

[IEvangelist]

This looks good to me. Let's have @eerhardt verify it too.

[eerhardt]

LGTM