chore(generator): move read_environment_variables to _compat.py.j2

[hebaalazzeh]

We move _read_environment_variables and its associated unit tests into _compat.py.j2 as read_environment_variables.

This reduces code duplication in packages that have multiple services. In the future, once should_use_mtls_endpoint is added to google-auth, we can defer parsing GOOGLE_API_USE_MTLS_ENDPOINT upstream.

Towards: #17883

[gemini-code-assist]

Code Review

This pull request refactors the environment variable reading logic by moving _read_environment_variables from the service client template into a shared helper function read_environment_variables in _compat.py.j2, and adds corresponding unit tests. The review feedback highlights critical issues: a missing closing parenthesis in _compat.py.j2 that will cause a syntax error, and missing imports for MutualTLSChannelError in both _compat.py.j2 and test_compat.py.j2 which will lead to runtime NameErrors.

[gemini-code-assist]

MutualTLSChannelError is raised in read_environment_variables but is not imported in this file. Please import it from google.auth.exceptions to avoid a NameError at runtime.

    from google.auth.exceptions import MutualTLSChannelError

    use_client_cert = should_use_client_cert()

[gemini-code-assist]

MutualTLSChannelError is used in test_read_environment_variables but is not imported in this test file. Please import it from google.auth.exceptions to avoid a NameError during test execution.

from {{package_path}}._compat import get_universe_domain, get_api_endpoint, get_default_mtls_endpoint, should_use_client_cert, read_environment_variables
from google.auth.exceptions import MutualTLSChannelError