Python: support enable_message_events parameter in enable_instrumentation (#8412)

[Manohar Paturi (ManoharPaturi)]

Motivation & Context

1. Why is this change required?
   enable_instrumentation() in agent_framework.observability allowed configuring enable_sensitive_data and force, but did not accept enable_message_events. When using the Microsoft OpenTelemetry Distro (configure_azure_monitor), callers need to configure message event emission without mutating internal settings or relying solely on environment variables.
2. What problem does it solve?
   As described in [Feature]: Support enable_message_events parameter in enable_instrumentation() to allow Microsoft OTel Distro configuration #8412, configure_azure_monitor in the Microsoft OTel Distro has its own message event collection and can cause duplicate message events unless enable_message_events=False can be explicitly configured.
3. What scenario does it contribute to?
   Programmatic configuration of message event emission in enable_instrumentation() alongside existing enable_sensitive_data.
4. If it fixes an open issue, please link to the issue below.
   Closes [Feature]: Support enable_message_events parameter in enable_instrumentation() to allow Microsoft OTel Distro configuration #8412.

Description & Review Guide

• What are the major changes?
  • Added enable_message_events: bool | None = None keyword argument to enable_instrumentation().
  • When enable_message_events is not None, sets OBSERVABILITY_SETTINGS.enable_message_events = enable_message_events.
  • When None, re-reads ENABLE_MESSAGE_EVENTS from environment (defaulting to True) to handle environment changes after import (e.g. load_dotenv()), aligning with enable_sensitive_data.
  • Added docstrings and unit tests in test_observability.py.
• What is the impact of these changes?
  • Clean API parity for enable_message_events alongside enable_sensitive_data without breaking changes.
• What do you want reviewers to focus on?
  • Parameter defaults and behavior alignment with enable_sensitive_data.

Related Issue

Closes #8412

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

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[Eduard van Valkenburg (eavanvalkenburg)]

Closing, this is a duplicate of #8451