🐛 Fix incomplete model config handling

[original4422]

Summary

• return an empty model name for empty configuration mappings
• safely handle partial mappings that omit model_repo or model_name
• add regression coverage while preserving complete repo/name formatting

Root cause

TenantConfigManager.get_model_config can return an empty default mapping for invalid model IDs, but get_model_name_from_config indexed both keys directly. That mismatch raised KeyError before callers could handle the empty model name.

Testing

• python -m pytest test/backend/utils/test_config_utils.py -q (35 passed)
• python -m compileall -q backend/utils/config_utils.py test/backend/utils/test_config_utils.py
• uvx ruff check --select E9,F63,F7,F82 backend/utils/config_utils.py test/backend/utils/test_config_utils.py
• git diff --check

Fixes #3810

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