fix(test): register baidu_obs mock as pytest plugin

[lin-snow]

Fixes #35623

Summary

api/tests/unit_tests/oss/baidu_obs/test_baidu_obs.py (added in #34330) imports the mock fixture as a regular module-level symbol:

from tests.unit_tests.oss.__mock.baidu_obs import setup_baidu_obs_mock

That imports the function but does not register it as a pytest fixture, so any run that actually collects this file fails with fixture 'setup_baidu_obs_mock' not found and 6 ERROR entries (one per test in TestBaiduObs).

This PR replaces the unused import with the standard pytest_plugins declaration, matching test_aliyun_oss.py, test_tencent_cos.py and test_volcengine_tos.py:

pytest_plugins = ("tests.unit_tests.oss.__mock.baidu_obs",)

Why this slipped through CI: Run API Tests / API Unit Tests (3.12) is gated by a path filter. PR #34330 only touched files under api/tests/unit_tests/oss/baidu_obs/, which the filter does not recognize as an "API change", so the unit-test job was skipped on that PR and on every subsequent main push. The first PRs that touch broader api/ paths (e.g. #35515) hit the failure. Verified by inspecting the job conclusions on recent main runs: Run API Tests is skipped on every one of them.

Verification:

$ MOCK_SWITCH=true pytest tests/unit_tests/oss/baidu_obs/test_baidu_obs.py
======================== 7 passed, 2 warnings in 0.46s =========================

Screenshots

Before	After
ERROR ... fixture 'setup_baidu_obs_mock' not found (×6)	7 passed

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

Note on make type-check: the command surfaces several pre-existing errors on main (e.g. app_factory.py:132, services/model_load_balancing_service.py:623, several in providers/trace/trace-tencent/...) that are unrelated to this one-line test fix. Confirmed identical errors on upstream/main HEAD.