[agentserver] core: send Foundry-Features header on task-store requests

The Foundry Task Storage API requires opt-in to the 'Routines=V1Preview'
preview feature via the 'Foundry-Features' header. Without it the
server returns HTTP 403 with:

  {"error":{"code":"preview_feature_required",
    "message":"This operation requires the following opt-in preview
    feature(s): Routines=V1Preview. Include the 'Foundry-Features:
    Routines=V1Preview' header in your request."}}

Wired into the HostedTaskProvider's HeadersPolicy as a base header so
every task-store request (GET/POST/PATCH/DELETE/list) carries it.

Discovered while running the durable-agent-demo against a fresh
deployment of the spec 016 transport refactor.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RaviPidaparthi

sdk/agentserver/azure-ai-agentserver-core/azure/ai/agentserver/core/durable/_client.py

@@ -331,7 +331,7 @@ def _build_default_policies(
     """
     return [
         RequestIdPolicy(),
-        HeadersPolicy(),
+        HeadersPolicy(base_headers={"Foundry-Features": "Routines=V1Preview"}),
         UserAgentPolicy(base_user_agent=_USER_AGENT),
         # Retry on 5xx and the standard transient HTTP statuses; 409
         # is explicitly NOT in retry_on_status_codes (FR-030) because