test(#1339): add test for FBC with empty index_repo_map

[fullsend-ai-coder]

Add a fourth parametrize entry to test_overwrite_from_index covering the case where is_image_fbc=True but url_repo_map is empty ({}). This tests the guard added in PR #1338 (commit cad01fa) that skips get_catalog_dir and push_configs_to_git when the index_repo_map is empty (containerized IIB path where push was already done earlier).

Changes:

• Add parametrize case: FBC=True, empty url_repo_map, different
  registries (no OCI export)
• Update assertion condition from if is_image_fbc to
  if is_image_fbc and url_repo_map to match production guard
• Add mock_pcg.assert_not_called() in the else branch to verify
  push_configs_to_git is also skipped when appropriate

Note: Full pytest suite could not run due to missing krb5-dev system library (required by gssapi/requests-kerberos). Test logic was validated via isolated mock-based execution of all 4 cases.

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Closes #1339

Post-script verification

• Branch is not main/master (agent/1339-test-fbc-empty-map)
• Secret scan passed (gitleaks — cad01fa661edff773b3c3e340fb5a8865906dcaf..HEAD)
• Pre-commit hooks passed (authoritative run on runner)
• Tests ran inside sandbox

[fullsend-ai-review]

Review

Findings

Info

• [test-integrity] tests/test_workers/test_tasks/test_build.py:469 — Pre-existing duplicate assertion: mock_pcg.assert_called_once_with(...) appears twice in sequence (lines 462–468 and 469–475). This is redundant but harmless — the second call re-verifies the same condition. Not introduced by this PR.

• [test-adequacy] tests/test_workers/test_tasks/test_build.py:489 — The addition of mock_pcg.assert_not_called() in the else branch is a good improvement. It strengthens coverage for both existing case 2 (is_image_fbc=False) and new case 4 (is_image_fbc=True, url_repo_map={}), ensuring push_configs_to_git is not called when the FBC-with-repo-map path is skipped.

No blocking findings. The new parametrize case correctly exercises the is_image_fbc=True with empty url_repo_map scenario, the assertion condition change accurately mirrors the production guard added in PR #1338, and the new negative assertion strengthens test coverage.