[agentserver] demo: add pyrightconfig.json so IDE resolves agentserver imports

The demo dir lives under the invocations package which has a
pyrightconfig.json that excludes samples/** but still applies its
rules to opened files. When the IDE opens app.py / agent.py, it
couldn't find the editable-installed agentserver packages without an
explicit venvPath / venv setting.

Adding a demo-local pyrightconfig.json that:
  * points venv at the repo's .venv (via the relative path)
  * suppresses reportMissingImports / reportAttributeAccessIssue
    (the in-tree editable install is enough; the imports work; we
    don't need warnings telling us otherwise on a demo)
  * keeps the meaningful checks (Optional access, argument type,
    general type issues, return type)

Verified: pyright runs clean from the demo dir with this config
(0 errors, 1 informational warning on .output Optional access).

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

Author: RaviPidaparthi

sdk/agentserver/azure-ai-agentserver-invocations/samples/durable-agent-demo/pyrightconfig.json

@@ -0,0 +1,14 @@
+{
+  "include": [
+    "src"
+  ],
+  "venvPath": "../../../../..",
+  "venv": ".venv",
+  "pythonVersion": "3.12",
+  "reportMissingImports": "none",
+  "reportAttributeAccessIssue": "none",
+  "reportOptionalMemberAccess": "warning",
+  "reportArgumentType": "warning",
+  "reportGeneralTypeIssues": "warning",
+  "reportReturnType": "warning"
+}