[MCP/Service Labels] Add service label workflow to REST API Specs

The Azure SDK Agent, in the context of a chat in the specs repo, can check for the existence of a label for a given service and, if no label exists, propose a label to the user and initiate the process of getting the label approved before a PR in the SDK repo is opened.

## Read labels, propose a label

see https://github.com/Azure/azure-sdk-tools/issues/11258

## No label exists, open a PR

see #11267

## Output to other agents

Store proposed label in a place where later agents can pick up the output and use it as part of a PR process. Values produced here may not be valid and may be updated in other places.

Start with linked issues 11258 and 11267, then locate the REST API Specs workflow or Azure SDK Agent entry point in this repository. Trace how service labels are checked and how agent output is stored; done means a missing label can be proposed, approval can be initiated before an SDK PR, and later agents can consume the stored value.

Written by the indexing model from the issue text.