Deploy task can't parse SOAP Response

[yippie]

Describe the bug

When using the Deploy task to deploy code to production, at least if there are unit test errors, instead of the error, we get the following:
Error: Could not process MDAPI response:

Reproduction steps

1. Have a deploy set to run unit tests that will fail
2. Set up deploy task with rest_deploy: False
3. Deploy to the org
4. EXPECTED: Task to fail with the list of failed unit tests
5. ACTUAL: Task fails with Error: Could not process MDAPI response: and raw soap envelope XML

Your CumulusCI and Python versions

CumulusCI version: 4.6.0 (/usr/local/bin/cci)
Python version: 3.13.5 (/opt/pipx/venvs/cumulusci/bin/python)

Operating System

macOS 26.2

Windows environment

No response

CumulusCI installation method

None

Error Gist

No response

Additional information

No response