fix: Fixed WatsonX remote inference provider (#3801)

# What does this PR do?
This PR fixes issues with the WatsonX provider so it works correctly
with LiteLLM.

The main problem was that WatsonX requests failed because the provider
data validator didn’t properly handle the API key and project ID. This
was fixed by updating the WatsonXProviderDataValidator and ensuring the
provider data is loaded correctly.

The openai_chat_completion method was also updated to match the behavior
of other providers while adding WatsonX-specific fields like project_id.
It still calls await super().openai_chat_completion.__func__(self,
params) to keep the existing setup and tracing logic.

After these changes, WatsonX requests now run correctly.

## Test Plan
The changes were tested by running chat completion requests and
confirming that credentials and project parameters are passed correctly.
I have tested with my WatsonX credentials, by using the cli with `uv run
llama-stack-client inference chat-completion --session`

---------

Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/remote/inference/watsonx/config.py

@@ -7,18 +7,18 @@
 import os
 from typing import Any
 
-from pydantic import BaseModel, ConfigDict, Field
+from pydantic import BaseModel, Field
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack.schema_utils import json_schema_type
 
 
 class WatsonXProviderDataValidator(BaseModel):
-    model_config = ConfigDict(
-        from_attributes=True,
-        extra="forbid",
+    watsonx_project_id: str | None = Field(
+        default=None,
+        description="IBM WatsonX project ID",
     )
-    watsonx_api_key: str | None
+    watsonx_api_key: str | None = None
 
 
 @json_schema_type