Fix broken cross-reference links in message_mapper.py docstrings #241

The DocFx-generated API page for message_mapper shows unresolved xref placeholders instead of links:

<xref:InputMessages>
<xref:OutputMessages>

Root cause: The docstrings in libraries/microsoft-agents-a365-observability-extensions-agentframework/microsoft_agents_a365/observability/extensions/agentframework/message_mapper.py use short :class: references (e.g., :class:InputMessages``). DocFx converts these to <xref:InputMessages>, which it cannot resolve because `InputMessages` is not defined in this module — it is imported from `microsoft_agents_a365.observability.core.models.messages`.

Fix required: Replace the short :class: references with fully-qualified names so DocFx can resolve them to their definition site. No logic changes are needed — this is a docstring-only fix.