feat: add dynamic model registration support to TGI inference

add new overwrite_completion_id feature to OpenAIMixin to deal with TGI always returning id=""

test with -

tgi: `docker run --gpus all --shm-size 1g -p 8080:80 -v /data:/data ghcr.io/huggingface/text-generation-inference --model-id Qwen/Qwen3-0.6B`

stack: `TGI_URL=http://localhost:8080 uv run llama stack build --image-type venv --distro ci-tests --run`

test: `./scripts/integration-tests.sh --stack-config http://localhost:8321 --setup tgi --subdirs inference --pattern openai`

tests/integration/suites.py

@@ -90,6 +90,16 @@ SETUP_DEFINITIONS: dict[str, Setup] = {
             "embedding_model": "sentence-transformers/all-MiniLM-L6-v2",
         },
     ),
+    "tgi": Setup(
+        name="tgi",
+        description="Text Generation Inference (TGI) provider with a text model",
+        env={
+            "TGI_URL": "http://localhost:8080",
+        },
+        defaults={
+            "text_model": "tgi/Qwen/Qwen3-0.6B",
+        },
+    ),
 }