fix(community): correct inverted GOOGLE_APPLICATION_CREDENTIALS check in GoogleDriveLoader

[Humphrey (HumphreySun98)]

Description

GoogleDriveLoader._load_credentials had the ADC check inverted: it called
google.auth.default() when GOOGLE_APPLICATION_CREDENTIALS was not set,
and only triggered the InstalledAppFlow desktop-OAuth path when ADC was
configured. This is backwards: default() is the helper that requires ADC,
and InstalledAppFlow is the helper that consumes the user's credentials.json.

As reported in #1751, users who follow the documented desktop-OAuth quickstart
(drop in credentials.json, no env var) crash with:

Your default credentials were not found. To set up Application Default
Credentials, see https://cloud.google.com/docs/authentication/external/set-up-adc

This PR swaps the condition so each branch matches the helper it calls, and
adds two regression tests that pin the routing for both env-var states.

Relevant issues

Fixes #1751

Type

🐛 Bug Fix

Changes

• libs/community/langchain_google_community/drive.py: change
  elif "GOOGLE_APPLICATION_CREDENTIALS" not in os.environ: to
  elif "GOOGLE_APPLICATION_CREDENTIALS" in os.environ: so the ADC branch runs
  only when ADC is configured.
• libs/community/tests/unit_tests/test_drive.py: add
  test_load_credentials_uses_installed_app_flow_when_adc_not_set and
  test_load_credentials_uses_default_when_adc_set.

Testing

$ python -m pytest libs/community/tests/unit_tests/test_drive.py -v
...
test_load_credentials_uses_installed_app_flow_when_adc_not_set PASSED
test_load_credentials_uses_default_when_adc_set                PASSED
======================= 10 passed in 5.11s =======================

Reverting only the one-line drive.py change while keeping the new tests makes
both new tests fail, confirming they actually pin the buggy path.

ruff check and ruff format --check pass on both files.

Note

The fix is intentionally minimal — a one-token swap that matches the existing
comment at the call site ("# no need to write to file" for the ADC branch,
which only makes sense if that branch corresponds to ADC). No behavior changes
in the service-account / cached-token / passed-in-credentials paths above.

---

Disclaimer: this PR was prepared with the assistance of an AI agent (Claude Code). All code and test changes were reviewed by the author before submission.

[Humphrey (HumphreySun98)]

Heads-up for reviewers: the langchain-google-community-us (llm-integration-tests) red mark is unrelated to this PR.

The Cloud Build log shows 4 failures, all in tests/integration_tests/test_sheets_integration.py, all the same error:

HttpError 403: API_KEY_SERVICE_BLOCKED
"Requests to this API sheets.googleapis.com method ... are blocked"
consumer: projects/546814358370

That is the Sheets API being blocked at the GCP project level (llm-integration-tests), not anything this PR touches — this PR only changes a single token in drive.py and adds drive-only unit tests. The same check shows FAILURE on a prior external community PR (#1721), which was still merged. Unit tests, lint, and compile-integration are all green here.

Disclaimer: this PR was prepared with the assistance of an AI agent (Claude Code); all code and test changes were reviewed by the author before submission.