🧪 Add regression coverage for incomplete model configs

[original4422]

Summary

The safe missing-key handling from #3810 is already upstream, now in management.services.model.names. This PR preserves that implementation and its modelName fallback while retaining the regression coverage.

 get_model_name_from_config tests
+ empty, null and partial mappings
+ quick-config modelName fallback
+ model_name precedence over modelName

Evidence

• Before: current develop covers only the two complete model mappings in this test class.
• After: the parameterized regression matrix covers ten additional inputs. Against develop at 9ee52bce, the cumulative change is only this test matrix and the required copyright header in test_config_utils.py; backend and SDK production files are unchanged.
• backend/.venv/bin/python -m pytest test/backend/utils/test_config_utils.py test/backend/utils/test_model_name_utils.py -q: 48 passed.
• Focused ruff check --select E9,F63,F7,F82, compileall, the repository copyright-header checker, and current-base-relative git diff --check pass.
• Validation uses the retained original Python 3.11.16 backend environment and editable sdk[dev]; backend/SDK dependency manifests have not changed since the documented setup. This is unit-regression validation, not a full backend or live-service test claim.

Related to #3810. The original production fix and later helper relocation are preserved. This maintenance syncs the existing tests with current develop; it introduces no product behavior or formal-case changes.

Merge Danger

Door: two-way

Blast Radius: tests

The production helper matches current develop; the effective contribution is regression coverage only.

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[original4422]

Hi @WMC001, when you have a chance, could you please take a look and let me know whether this is ready to merge or needs any changes? Thank you.

[original4422]

Hi @WMC001, when convenient, could you please check whether this PR is ready to merge or needs any changes? Thank you!

[original4422]

Hi @WMC001, when you have a moment, could you please check whether this PR needs any changes before it can be merged? Thank you!

[original4422]

Hi @WMC001, when convenient, could you let me know whether this PR needs any changes before merge? Thank you!

[original4422]

Hi @jeffwu-1999, when convenient, could you please check whether this PR needs any changes before merge? Thank you!

[original4422]

Updated in 18f4839 to preserve the upstream modelName fallback and extend regression coverage for empty/partial mappings and name precedence. The branch passes all 43 tests in test_config_utils.py; its prospective merge with current develop matches the locally validated tree with 44 passing tests. Focused Ruff, compileall and whitespace checks also pass. The effective merged change adds tests only; the existing production behavior is preserved.

[original4422]

Hi @WMC001, when you have a chance, could you please review the updated regression coverage and let me know whether this is ready to merge? Thank you!

[original4422]

Hi @YehongPan, when you have a chance, could you please review the regression coverage for incomplete model configs and let me know if any changes are needed? Thank you!

[original4422]

Synced with current develop (9ee52bce) in b56c5ea631b44664a684b03cdc0aab24b7fa32d2. The relocated production helper remains unchanged; the cumulative PR now contains only the existing incomplete-config regression matrix and the required copyright header in its test file. The original config-utils and model-name-utils suites pass 48 tests; focused Ruff, compileall, the copyright checker and cumulative whitespace checks also pass. Both newly published maintenance commits have valid GitHub-verified signatures. This sync does not claim full backend or live-service validation.

[original4422]

Hi @WMC001, when convenient, could you please review the updated regression coverage for incomplete model configs and let me know if any changes are needed? Thank you.