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Summary of the issue

Context
google.auth.default() will use http.client as default transport, but mTLS support needs session support to work

Expected Behavior:
return proper project_id with default transport

Actual Behavior:
returned project_id is None

API client name and version

google-auth 2.49.0

Reproduction steps: code

file: main.py

import google.auth
credentials, project_id = google.auth.default()
assert(project_id)

Worked in 2.43.0 and broke with 2.44.0 and later due mTLS support, which has a hard requirement for requests.

Workaround:
file: main.py

import google.auth
from google.auth.transport.requests import Request
credentials, project_id = google.auth.default(request=Request())
assert(project_id)

Reproduction steps: supporting files

No response

Reproduction steps: actual results

No response

Reproduction steps: expected results

No response

OS & version + platform

Debian 12

Python environment

3.11.2

Python dependencies

No response

Additional context

No response