Python: expose enable_message_events through enable_instrumentation #8443

Scope

Python implementation of #8412. Microsoft OTel Distro needs to call enable_instrumentation(enable_message_events=False) without configuring or replacing Agent Framework's OpenTelemetry providers/exporters.

On upstream/main at 999dda7970fe969c0901d524365ce34ecbda6227, enable_instrumentation accepts only enable_sensitive_data and force; configure_otel_providers already supports enable_message_events.

Acceptance criteria

• Add the keyword-only enable_message_events: bool | None = None parameter to the public Python enable_instrumentation API.
• Explicit True/False sets the existing message-event control, overriding the value previously obtained from the environment or provider configuration.
• Omitted/None leaves the current setting unchanged, preserving backward compatibility and prior configuration.
• Preserve existing sensitive-data gating, sticky disable unless force=True, and external provider/exporter ownership.
• Add focused regression coverage for the public call, explicit/omitted values, environment precedence, disable/force behavior, actual message emission, and provider preservation; update directly relevant documentation and any required API baseline.

Boundaries

No .NET changes, observability redesign, or unrelated span-metadata fixes. A Python PR should close this subissue and reference #8412 as partially addressed, without closing the parent.