.NET: track programmatic message-event configuration for OTel Distro

## Scope

Track the .NET portion of microsoft/agent-framework#8412, which is labeled Language/SDK: Both. Python implementation is tracked separately in microsoft/agent-framework#8443.

The concrete `enable_instrumentation(enable_message_events=False)` API and verified missing parameter belong to Python. An equivalent missing .NET API has not yet been established; this issue tracks that investigation and any necessary .NET implementation rather than assuming the Python API maps directly to .NET.

## Acceptance criteria

- Confirm the applicable .NET instrumentation entry point and Microsoft OTel Distro integration requirement with maintainers.
- Determine whether existing .NET controls already support programmatic suppression of message events without replacing externally owned providers/exporters.
- If a gap exists, implement the smallest additive .NET API/configuration change, preserving existing defaults, sensitive-data controls, and provider ownership, with focused tests and documentation.
- If no .NET change is needed, record the supported mechanism and evidence so the parent issue can be resolved accurately.

## Boundaries

Do not duplicate the Python work in microsoft/agent-framework#8443 or include unrelated observability/span-metadata fixes. A .NET PR should close this subissue only when its established requirements are fully addressed; the parent remains open until both language tracks are resolved.

Start by confirming the applicable .NET instrumentation entry point and Microsoft OTel Distro integration requirement with maintainers. Inspect existing .NET controls for message-event suppression, provider ownership, defaults, and sensitive-data handling. Done means either documenting the supported mechanism with evidence or adding the smallest focused API/configuration change with tests and documentation.

Written by the indexing model from the issue text.