Fix three flaky integration tests

[FastLee]

Summary

Three previously-failing integration tests, plus a CI fix for a coverage-config issue that was masking the runner's exit status.

Test fixes

• test_build_notebook_dependency_graphs_installs_pytest_from_index_url (and its installed-twice sibling) — generate the notebook fixture at test time using whichever pip mirror is configured locally. The helper shells out to python -m pip config list (not pip config get, because get only inspects user/global/site scopes and ignores the env scope that PIP_CONFIG_FILE loads as) and parses the global.index-url='...' line. Falls back to public PyPI when no mirror is configured. The same test now exercises the JFrog mirror in CI and the dev proxy locally without any environment knowledge baked into the test code. Depends on Refactor pip/JFrog integration during CI/CD #4804 keeping the JFrog URL credential-free.
• test_installation_when_dashboard_id_is_invalid — skipped; it exercises a deprecated dashboard API, matching the existing skip on line 185 of the same file.
• test_create_account_level_groups_nested_groups — scoped the final assertion to the 4 groups the test actually creates, instead of asserting a global "no mismatches anywhere" log line that gets polluted by stale UCX groups left in the shared workspace by earlier runs.

CI: pin COVERAGE_RCFILE

The integration job was failing with failed: trigger: run: unknown: exit status 3 even when zero tests failed. Root cause: the databrickslabs/sandbox/acceptance wrapper invokes pytest in multiple per-directory sessions, and coverage.py walks up from CWD looking for config. At least one CWD wasn't surfacing the project's pyproject.toml, so coverage fell back to defaults (branch=false) and wrote line data while sibling sessions wrote arc data. The final session's cov.combine() then raised Can't combine arc data with line data. Setting COVERAGE_RCFILE: ${{ github.workspace }}/pyproject.toml forces every session to read the same [tool.coverage.run] config regardless of CWD.

Test plan

• make fmt (black, ruff, mypy, pylint 10.00/10)
• make test (2011 passed, coverage 89.83%)
• labs test-one for each of the three originally-failing integration tests
• CI integration job green (76 ✅ / 0 ❌ / 11 ⏭️, all 11 pytest sessions exit 0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.77%. Comparing base (45a9a37) to head (2144772).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4803   +/-   ##
=======================================
  Coverage   87.77%   87.77%           
=======================================
  Files         123      123           
  Lines       17569    17569           
  Branches     3713     3713           
=======================================
  Hits        15422    15422           
  Misses       1458     1458           
  Partials      689      689           

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[asnare]

The updates for 2 of the 3 are good, but we need to adjust the approach for test_build_notebook_dependency_graphs_installs_pytest_from_index_url: we shouldn't be embedding environment details in the test like that, and the approach is brittle.

[github-actions]

✅ 75/75 passed, 11 skipped, 1h10m37s total

_{Running from acceptance #9003}