feat(sdk): Add native credential remediation agent ('Merge Assurance')

[minhkhoango]

Why this matters

Expired API credentials are a common cause of silent integration failures. For Merge users, they are frustrating and expensive. Keyfactor's 2024 report mentions that a single outage from an expired machine identity is 2.86 million

My proposal: Merge Assurance Agent

This pull request adds Merge Assurance—a built-in, self-healing agent for the Python SDK. You can turn it on with a single line: client.remediation.enable_assurance().

• The agent runs natively, so credentials never leave your system.
• It's production-ready, outputting structured JSON logs, safe shutdown, and easy integration.
• It's resilient, with smart retry logic to handle network issues.

How to try it

Unit tests:

poetry install --with dev
pytest tests/remediation/

End-to-end demo:

`LOG_LEVEL=INFO poetry run python e2e_test/sample_app.py`

You'll see the agent detect, refresh, and report on credential status in real time. You can change LOG_LEVEL to DEBUG to WARNING.

Next steps

This is a production-ready proof of concept. I'd love your feedback and am happy to discuss how to make this a seamless part of the SDK.

Kai