[docs] Document unified withEnvironment API and IExpressionValue for multi-language AppHosts (Aspire 13.3)

src/frontend/src/content/docs/architecture/resource-api-patterns.mdx

@@ -156,9 +156,11 @@ Custom value objects defer evaluation and allow the framework to discover depend
 |--|--|--|--|
 | `IValueProvider` | `ValueTask<string?> GetValueAsync(CancellationToken)` | Run | Resolve live values at runtime |
 | `IManifestExpressionProvider` | `string ValueExpression { get; }` | Publish | Emit structured expressions in manifests |
+| `IExpressionValue` | Inherits `IValueProvider` and `IManifestExpressionProvider` | Run and publish | Mark a value object as usable wherever an expression-backed value is accepted |
 | `IValueWithReferences` _(opt.)_ | `IEnumerable<object> References { get; }` | Both (if needed) | Declare dependencies on other resources |
 
 - **Implement** `IValueProvider` and `IManifestExpressionProvider` on all structured value types.  
+- **Implement** `IExpressionValue` when a structured value type should be accepted by APIs such as `WithEnvironment(...)`.
 - **Implement** `IValueWithReferences` only when your type holds resource references.
 
 ### Attaching to resources
@@ -198,5 +200,6 @@ public static IResourceBuilder<T> WithEnvironment<T>(
 |--|--|--|
 | `IValueProvider` | `GetValueAsync(...)` | Deferred runtime resolution |
 | `IManifestExpressionProvider` | `ValueExpression` | Structured publish-time expression |
+| `IExpressionValue` | `IValueProvider` + `IManifestExpressionProvider` | Reusable expression-backed value |
 | `IValueWithReferences` _(opt.)_ | `References` | Declare resource dependencies |
-| `WithEnvironment(...)` | `new("NAME", valueProvider)` | Attach structured values unflattened |
+| `WithEnvironment(...)` | `new("NAME", valueProvider)` | Attach structured values unflattened |

src/frontend/src/content/docs/extensibility/multi-language-integration-authoring.mdx

@@ -479,7 +479,13 @@ When a parameter accepts multiple types, use `[AspireUnion]` to declare the vali
 public static IResourceBuilder<T> WithEnvironment<T>(
     this IResourceBuilder<T> builder,
     string name,
-    [AspireUnion(typeof(string), typeof(ReferenceExpression), typeof(EndpointReference))]
+    [AspireUnion(
+        typeof(string),
+        typeof(ReferenceExpression),
+        typeof(EndpointReference),
+        typeof(IResourceBuilder<ParameterResource>),
+        typeof(IResourceBuilder<IResourceWithConnectionString>),
+        typeof(IExpressionValue))]
     object value)
     where T : IResourceWithEnvironment
 {
@@ -596,7 +602,7 @@ The following types are ATS-compatible and can be used in exported method signat
 | **Collections** | `List<T>`, `Dictionary<string, T>`, arrays — where `T` is ATS-compatible                             |
 | **Delegates**   | `Action<T>`, `Func<T>`, and other delegate types (use `RunSyncOnBackgroundThread = true` for synchronous delegates invoked inline) |
 | **Services**    | `ILogger`, `IServiceProvider`, `IConfiguration` (already exported by the core framework)             |
-| **Special**     | `ParameterResource`, `ReferenceExpression`, `EndpointReference`, `CancellationToken`                 |
+| **Special**     | `ParameterResource`, `ReferenceExpression`, `EndpointReference`, `IExpressionValue`, `CancellationToken` |
 | **Nullable**    | Any of the above as nullable (`T?`)                                                                  |
 
 Types that are **not** ATS-compatible include: interpolated string handlers and custom complex types without `[AspireExport]` or `[AspireDto]`.

src/frontend/src/content/docs/whats-new/aspire-13-3.mdx

@@ -137,6 +137,36 @@ You can also disable it in `launchSettings.json`:
   For more details on container networking, see [Inner-loop networking overview](/fundamentals/networking-overview/).
 </LearnMore>
 
+## 🔀 Unified withEnvironment API for multi-language AppHosts
+
+Aspire 13.3 introduces a unified `withEnvironment(name, value)` API for polyglot AppHosts (TypeScript, Java, Python, Go, Rust). Previously, environment variable injection required separate methods for each value kind (`withEnvironmentEndpoint`, `withEnvironmentParameter`, `withEnvironmentConnectionString`, and so on). Now, a single call handles all value types:
+
+```typescript title="TypeScript — apphost.ts"
+const api = await builder.addApi("api")
+    .withEnvironment("SERVICE_URL", cache.primaryEndpoint)
+    .withEnvironment("API_KEY", apiKeyParam)
+    .withEnvironment("DB", database);
+```
+
+The `value` argument accepts any of: a plain `string`, a `ReferenceExpression`, an `EndpointReference`, a parameter builder, a connection string resource builder, or an `IExpressionValue` — a new public interface that any type implementing both `IValueProvider` and `IManifestExpressionProvider` can implement to plug into this unified path.
+
+### Deprecation of old withEnvironment* aliases
+
+The previous per-kind aliases are still generated for backward compatibility but are marked `@deprecated` in the SDK. Migrate to `withEnvironment(name, value)` at your convenience:
+
+| Deprecated | Replacement |
+|---|---|
+| `withEnvironmentExpression` | `withEnvironment` |
+| `withEnvironmentEndpoint` | `withEnvironment` |
+| `withEnvironmentParameter` | `withEnvironment` |
+| `withEnvironmentConnectionString` | `withEnvironment` |
+| `withEnvironmentFromOutput` | `withEnvironment` |
+| `withEnvironmentFromKeyVaultSecret` | `withEnvironment` |
+
+<LearnMore>
+  For integration authors, see [Multi-language integrations](/extensibility/multi-language-integration-authoring/#union-types) for union-type parameter annotations.
+</LearnMore>
+
 ## 🐍 Python starter template migrated to TypeScript AppHost
 
 The `aspire-py-starter` template (Starter App with FastAPI and React) has moved from the `dotnet new` template system to the Aspire CLI template system and now uses a **TypeScript AppHost** instead of a C# AppHost. This aligns it with the same pattern as the `aspire-ts-starter` template.