Fix verbose source attribution when falling back to INI env

[shuofengzhang]

What changed

• Fixed verbose source attribution so --pytest-env-verbose reports the INI file when env entries come from INI fallback.
• Added a regression test for the case where a pyproject.toml exists but does not contain a pytest_env section.
• Tightened the existing INI verbose test to assert the reported source path.

Why

• Previously, if a TOML file was discovered but had no pytest_env entries, verbose output still labeled inline env assignments as coming from that TOML file.
• This made debug output misleading for projects that keep build metadata in pyproject.toml while configuring env in pytest.ini.

Testing

• ✅ . .venv/bin/activate && pytest (71 passed)