chore: remove vision model URL workarounds and simplify client creation (#2775)

The vision models are now available at the standard URL, so the
workaround code has been removed. This also simplifies the codebase by
eliminating the need for per-model client caching.

- Remove special URL handling for meta/llama-3.2-11b/90b-vision-instruct
models
- Convert _get_client method to _client property for cleaner API
- Remove unnecessary lru_cache decorator and functools import
- Simplify client creation logic to use single base URL for all models

tests/unit/providers/nvidia/test_supervised_fine_tuning.py

@@ -54,7 +54,8 @@ class TestNvidiaPostTraining(unittest.TestCase):
         self.mock_client.chat.completions.create = unittest.mock.AsyncMock()
         self.inference_mock_make_request = self.mock_client.chat.completions.create
         self.inference_make_request_patcher = patch(
-            "llama_stack.providers.remote.inference.nvidia.nvidia.NVIDIAInferenceAdapter._get_client",
+            "llama_stack.providers.remote.inference.nvidia.nvidia.NVIDIAInferenceAdapter._client",
+            new_callable=unittest.mock.PropertyMock,
             return_value=self.mock_client,
         )
         self.inference_make_request_patcher.start()