ExtensionInformationAttribute should support multiple extensions per assembly

### Description

## Problem

`ExtensionInformationAttribute` has [`AllowMultiple` defaulting to `false`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/extensions/Worker.Extensions.Abstractions/src/ExtensionInformationAttribute.cs#L9), so a library assembly can only declare **one** host extension dependency. Libraries that need to auto-load multiple host extensions cannot do so today.

This is a problem when a library dynamically registers functions at runtime (via `IFunctionMetadataProvider`) for multiple extension types — the build-time SDK scan won't detect those bindings, so `ExtensionInformationAttribute` with `enableImplicitRegistration: true` is the only way to force-load the host extensions. Being limited to one attribute forces library authors to choose which single extension to auto-load.

### Example

[`Microsoft.Agents.AI.Hosting.AzureFunctions`](https://github.com/microsoft/agent-framework/blob/b298113d15cf5c7e8a7299991cea98ef0400dbbf/dotnet/src/Microsoft.Agents.AI.Hosting.AzureFunctions/Microsoft.Agents.AI.Hosting.AzureFunctions.csproj#L43-L56) dynamically registers functions for both DurableTask and MCP extensions, but can only declare one `ExtensionInformationAttribute`.

## Background

During `dotnet build`, the Worker SDK scans all NuGet package assemblies for `ExtensionInformationAttribute` to discover host extensions. The collected references are used to generate `WorkerExtensions.csproj`, which is built to produce `extensions.json`. At runtime, the Functions host reads `extensions.json` to load host extensions — if an extension is missing, it won't load.

Relevant code path:
- [`CollectExtensionPackages`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/src/Azure.Functions.Sdk/Targets/Extensions/Azure.Functions.Sdk.Extensions.targets#L66-L73) — MSBuild target that triggers extension discovery after restore
- [`ResolveExtensionPackages`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/src/Azure.Functions.Sdk/Tasks/Extensions/ResolveExtensionPackages.cs) — MSBuild task that iterates package assemblies
- [`ExtensionReference.TryGetFromModule`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/src/Azure.Functions.Sdk/ExtensionReference.cs#L28-L55) — reads the attribute from an assembly via Mono.Cecil
- [`ExtensionInformationAttribute`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/extensions/Worker.Extensions.Abstractions/src/ExtensionInformationAttribute.cs#L9) — the attribute definition

There are **two** constraints preventing multiple extensions per assembly:

1. The attribute has [`AllowMultiple = false`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/extensions/Worker.Extensions.Abstractions/src/ExtensionInformationAttribute.cs#L9) (compile-time block)
2. `TryGetFromModule` [`return true` after first match](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/src/Azure.Functions.Sdk/ExtensionReference.cs#L52) (scanner-level block)

## Proposed Changes

### 1. [`ExtensionInformationAttribute.cs`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/extensions/Worker.Extensions.Abstractions/src/ExtensionInformationAttribute.cs#L9)

```diff
- [AttributeUsage(AttributeTargets.Assembly)]
+ [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
public class ExtensionInformationAttribute : Attribute
```

### 2. [`ExtensionReference.TryGetFromModule`](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/src/Azure.Functions.Sdk/ExtensionReference.cs#L28-L55)

Change to return **all** matching attributes instead of stopping at first match. The [caller](https://github.com/Azure/azure-functions-dotnet-worker/blob/cec19d28cf81f943bce994deb276a2885c3a7804/src/Azure.Functions.Sdk/Tasks/Extensions/ResolveExtensionPackages.cs#L134-L160) already uses `yield return`, so handling multiple results is straightforward.

## Result

This enables a **single assembly** to declare multiple host extension dependencies via `ExtensionInformationAttribute`. Today, `extensions.json` already supports multiple extensions (each NuGet package assembly contributes its own), but a single assembly is limited to one. With this change, a library assembly can contribute multiple extensions to `extensions.json`.

This is **backwards-compatible**: assemblies with a single attribute continue to work as before. No changes needed for existing extension packages.