Add Foundry Local as a chat and embedding provider in the AI Chat Web template

### Summary

The AI Chat Web template (`dotnet new aichatweb`, `Microsoft.Extensions.AI.Templates`) lets
you pick a chat and embedding provider: Azure OpenAI, GitHub Models, OpenAI, and Ollama. The
only on-device option today is Ollama, a third-party runtime. This proposes adding
[Foundry Local](https://www.foundrylocal.ai/), Microsoft's own on-device inference runtime, as
a first-class provider for both the chat `IChatClient` and the embedding `IEmbeddingGenerator`.

### Why

- It gives the template a Microsoft-owned local inference story: no cloud, no API key, models
run on the developer's machine on CPU, GPU, or NPU through ONNX Runtime.
- Foundry Local is OpenAI-compatible, so it drops into the same
`OpenAIClient -> AsIChatClient() / AsIEmbeddingGenerator()` shape the template already uses
for the OpenAI and GitHub Models providers. The change is small and well-contained.
- The template's provider list is the de facto answer to "what does .NET AI support locally."
Adding Foundry Local closes the gap and gives the docs and RAG tutorials a reference
integration to point at.

### What it would look like

A new provider choice, for example `--provider foundrylocal`, that wires Foundry Local for
both chat and embeddings. Suggested defaults: `qwen3-4b` for chat (Apache-2.0, tool-calling
capable) and `qwen3-embedding-0.6b` for embeddings (1024-dimension vectors).

### Proof of concept

A runnable end-to-end sample is here:
https://github.com/luisquintanilla/foundry-local-aichatweb

It is the standard AI Chat Web app (Blazor, RAG over a local PDF, SQLite vector store) with the
provider swapped to Foundry Local. It builds and runs end to end, and includes a dev container
so you can try it in Codespaces.

An `aspire` branch shows what an Aspire orchestration could look like, using a small custom
`AddFoundryLocal` hosting integration (there is no Aspire hosting integration for Foundry Local
today): https://github.com/luisquintanilla/foundry-local-aichatweb/tree/aspire

### Notes for implementation

- SDK: `Microsoft.AI.Foundry.Local` 1.2.3 (stable, on nuget.org). The app uses the manager to
start the local OpenAI-compatible web service and load models, then points the standard
OpenAI client at it. Foundry Local is keyless on localhost.
- The embedding model returns 1024-dimension vectors, so `IngestedChunk.VectorDimensions`
(currently hardcoded to 1536) needs to become per-provider.
- First run downloads the model weights; later runs start fast.

### Scope

Non-Aspire first. The Aspire path is under investigation: there is no Aspire hosting
integration for Foundry Local today (unlike Ollama's `AddOllama`), so a custom
`AddFoundryLocal` integration is prototyped on the sample's `aspire` branch (linked above). We
can decide how to bring Aspire into the template once that settles.