test(github-projects): cover GraphQL data paths

[codeforester]

Summary

• Add direct hermetic coverage for GitHub Project GraphQL queries, mutations, pagination, and missing-resource handling.
• Exercise Project item field copying/defaulting and GraphQL transport/auth error classification.
• Raise base_github_projects coverage to 88.8% statements and 81.2% branches, above the issue's 85%/75% targets.

Issue

Fixes #2049

Validation

• python -m pytest cli/python/base_github_projects/tests/test_graphql_data_access.py cli/python/base_github_projects/tests/test_project_item_fields_graphql.py cli/python/base_github_projects/tests/test_subprocess_timeouts.py -q (32 passed)
• python -m pylint cli/python/base_github_projects/tests/test_graphql_data_access.py cli/python/base_github_projects/tests/test_project_item_fields_graphql.py cli/python/base_github_projects/tests/test_subprocess_timeouts.py
• python -m pytest cli/python/base_github_projects/tests --cov=cli/python/base_github_projects --cov-report=json:/private/tmp/base-2049-coverage.json -q (92 passed; 88.8% statements, 81.2% branches)
• ./bin/base-test with supported sibling source overrides (1,079 Python tests and 899 BATS tests passed)

Demo Impact

None. Test-only hardening; no user-facing behavior changes.

Notes

AI context and documentation are not applicable because this change only adds tests for existing internal contracts.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug>, and the prefix matches the issue's single standard category label.
• PR is scoped to one issue, unless a documented multi-issue exception applies.
• PR body explains what changed and how it was validated.
• Validation commands were run from the current checkout or worktree, or unavailable checks are explained.
• Relevant BATS and Python tests pass.
• Bug fixes include regression proof or a documented reproduction when practical.
• Documentation is updated when behavior or user-facing commands change.
• AI context is updated in .ai-context/, or the PR body explains why it is not applicable.
• PR includes Fixes #<issue> or Closes #<issue> when it should close the issue.
• Demo Impact is meaningful for needs-demo work, or explicitly says None.