Add --soft-fail option to capture Mypy output without blocking CI

Feature

Add a --soft-fail option to Mypy that allows capturing its output without causing a hard failure in CI/CD pipelines.

Pitch

Currently, Mypy exits with a 1 status code when type errors are found, which is expected behavior. However, in many CI/CD pipelines (especially those integrating with SonarQube or similar tools), it's useful to capture all Mypy output while still allowing the pipeline to continue in a controlled way.

A --soft-fail option would:

• Allow Mypy to run normally and produce full output, including errors and warnings.
• Exit with status code 0 so that CI/CD jobs don’t automatically fail.
• Still allow external tools like SonarQube to analyze and report issues without interfering with the pipeline’s flow.

I currently have a one liner workaround in tox:

https://gist.github.com/balihb/0b64435c0e8f553791071cea4a897c91

[tox]
min_version = 4.20
envlist =
# renovate: datasource=docker depName=python
    3.13

[testenv]
skip_install = true
pass_env =
    FORCE_COLOR
    MYPY_*
    NO_COLOR
    PIP_*
    PYTHONUNBUFFERED
    PY_COLORS
    TERM
deps =
    -r requirements.txt
    -r requirements-mypy.txt
commands =
commands =
    python -c "import os, re, shlex, subprocess, sys; \
    from pathlib import Path; \
    result = subprocess.run(['mypy'] + shlex.split('{posargs:src}'), check=False, text=True, capture_output=True); \
    result.stdout.strip() and sys.stdout.write(result.stdout); \
    result.stderr.strip() and sys.stderr.write(result.stderr); \
    output = Path(os.getenv('MYPY_TOX_OUTPUT', default='.report/mypy/output.txt')); \
    output.parent.mkdir(parents=True, exist_ok=True); \
    output.write_text(re.compile(r'\x1b\\[[0-9;]*?m').sub('', result.stdout)); \
    sys.exit(1 if result.returncode == 2 else 0)"

but this is far from ideal.

Start by tracing Mypy’s command-line entry point and the existing path that determines its exit status after reporting errors. Add the --soft-fail behavior so output remains unchanged while the command exits 0, and verify the option through the relevant CLI tests.