secops-soar: clarify certifi setup failures

[Haihan-Jiang]

Summary

• keep the underlying SOAR request error so startup can distinguish TLS certificate failures from credential failures
• show a certifi/Install Certificates.command hint when fetching SOAR scopes fails because certificate verification failed
• add the same macOS certifi setup note to the top-level README

Before/after evidence

Before, the certifi setup failure was reported as a credentials problem:

Failed to fetch valid scopes from SOAR, please make sure you have configured the right SOAR credentials. Shutting down...

After this change, the certificate path produces a specific setup hint:

Failed to fetch valid scopes from SOAR because TLS certificate verification failed. If you are using the Python.org macOS installer, run the Install Certificates.command for your Python version, for example: `/Applications/Python\ 3.12/Install\ Certificates.command`. You can also point Python at certifi's CA bundle with `SSL_CERT_FILE=$(python -m certifi)`.

Tests

• python3 -m py_compile server/secops-soar/secops_soar_mcp/bindings.py server/secops-soar/secops_soar_mcp/http_client.py server/secops-soar/secops_soar_mcp/bindings_test.py
• PYTHONPATH=server/secops-soar /tmp/mcp-security-secops-venv/bin/python -m pytest -q server/secops-soar/secops_soar_mcp/bindings_test.py
• git diff --check

Fixes #191