[llm_bench] Fix text use_case in case of complex_model_types (openvinotoolkit#2874)

## Description
use_cases have become object, but for complex model types are remains
text, fix it

<!--- Jira ticket number (e.g., 123). Delete if there's no ticket. Don't
include full link or project name. -->
Ticket: CVS-174931

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Fixes #(issue)

## Checklist:
- [ ] Tests have been updated or added to cover the new code <!--- If
the change isn't maintenance related, update the tests at
https://github.com/openvinotoolkit/openvino.genai/tree/master/tests or
explain in the description why the tests don't need an update. -->
- [ ] This patch fully addresses the ticket. <!--- If follow-up pull
requests are needed, specify in description. -->
- [ ] I have made corresponding changes to the documentation

Author: sbalandi

tools/llm_bench/llm_bench_utils/get_use_case.py

@@ -50,13 +50,13 @@ def safe_json_load(file_path: Path) -> Optional[dict]:
 def resolve_complex_model_types(config):
     model_type = config.get("model_type").lower().replace('_', '-')
     if model_type == "gemma3":
-        return "visual_text_gen", model_type
+        return USE_CASES["visual_text_gen"][0], model_type
     if model_type == "gemma3-text":
-        return "text_gen", model_type
+        return USE_CASES["text_gen"][0], model_type
     if model_type in ["phi4mm", "phi4-multimodal"]:
-        return "visual_text_gen", model_type
+        return USE_CASES["visual_text_gen"][0], model_type
     if model_type == "llama4":
-        return "visual_text_gen", model_type
+        return USE_CASES["visual_text_gen"][0], model_type
     return None, None