Update environment variable format to polyglot-friendly pattern

[Copilot]

Fixes #5308

This PR updates all documentation to reflect the new polyglot-friendly environment variable format for service endpoints introduced in dotnet/aspire#12141.

Changes

The environment variable naming convention for service endpoints has been updated from the .NET-specific format to a more language-agnostic format that works seamlessly with JavaScript, Python, and other non-.NET technologies.

Before

Environment variables used the format services__{resourcename}__{endpointname}__{index}:

var apiservice = builder.AddProject<Projects.ApiService>("apiservice");
var frontend = builder.AddProject<Projects.Frontend>("frontend")
    .WithReference(apiservice);

This generated environment variables like:

• services__apiservice__http__0="http://localhost:5455"
• services__apiservice__https__0="https://localhost:7356"

After

Environment variables now use the format {RESOURCENAME}_{ENDPOINTNAME}:

The same code now generates:

• APISERVICE_HTTP="http://localhost:5455"
• APISERVICE_HTTPS="https://localhost:7356"

Updated Documentation

• fundamentals/app-host-overview.md - Updated service endpoint environment variable format explanation and examples
• get-started/build-aspire-apps-with-nodejs.md - Updated JavaScript integration examples (Angular, React, Vue)
• deployment/manifest-format.md - Updated manifest format examples
• deployment/azd/aca-deployment-azd-in-depth.md - Updated Azure Container Apps deployment examples
• fundamentals/orchestrate-resources.md - Updated external service examples
• extensibility/dev-tunnels-integration.md - Updated dev tunnels integration examples
• testing/snippets/ - Updated test assertions across all three test frameworks (xUnit, MSTest, NUnit)

The new format is uppercase, doesn't include an index for single endpoints, and follows a consistent pattern that makes it easier to use in polyglot scenarios where non-.NET applications need to consume Aspire services.

Original prompt

Please address #5308 - by considering the changes in dotnet/aspire#12141. Look for all places where we refer to the structure of the env var, for example services__*

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Internal previews

📄 File	🔗 Preview link
docs/deployment/azd/aca-deployment-azd-in-depth.md	docs/deployment/azd/aca-deployment-azd-in-depth
docs/deployment/manifest-format.md	docs/deployment/manifest-format
docs/extensibility/dev-tunnels-integration.md	docs/extensibility/dev-tunnels-integration
docs/fundamentals/app-host-overview.md	docs/fundamentals/app-host-overview
docs/fundamentals/orchestrate-resources.md	docs/fundamentals/orchestrate-resources
docs/get-started/build-aspire-apps-with-nodejs.md	docs/get-started/build-aspire-apps-with-nodejs