[Feature] Resolve `${VAR}` interpolation in `azure.yaml` `image:` after predeploy hooks run

### Summary

`${VAR}` interpolation in `azure.yaml` (including a service's `image:`) is resolved at **project-load time**, which is **before** predeploy hooks execute. This makes it impossible to point `image:` at an artifact a predeploy hook produces.

### What happens

Our predeploy hook builds a container and produces a fresh **immutable** tag (e.g. `dev-N.0.0`) via `az acr build`. We would like `image:` to reference that tag through an env var the hook sets:

```yaml
services:
agent:
image: ${AGENT_DEPLOYED_IMAGE}
```

But because interpolation happens before the hook runs, `${AGENT_DEPLOYED_IMAGE}` is empty/stale at the moment `azd` reads it — so `azd` can never deploy the just-built immutable tag.

### Current workaround

The predeploy hook performs the container repoint itself (`az ... config container set`) and writes the immutable tag back to `.env`, bypassing `azure.yaml`'s `image:` entirely.

### Ask

Support **late-resolved** `${VAR}` for fields like `image:` — resolve interpolation *after* predeploy hooks run — so a hook can hand `azd` the tag it just built.

---
_Context: encountered while deploying a multi-component AI/chat service (App Service + Function App + Logic App + Foundry hosted agent) end-to-end with `azd` provision/deploy in [Azure/azure-sdk-tools `tools/sdk-ai-bots/deployment`](https://github.com/Azure/azure-sdk-tools/tree/main/tools/sdk-ai-bots/deployment). We currently work around this with a TypeScript hook._

Start by tracing how azure.yaml interpolation is handled during the azd provision/deploy lifecycle and when predeploy hooks run. Review the TypeScript hook and its .env workaround in tools/sdk-ai-bots/deployment. Done means a hook-produced immutable image tag is available to the image field after the hook and is deployed by azd.

Written by the indexing model from the issue text.