feat!: standardize base_url for inference (#4177)

# What does this PR do?

Completes #3732 by removing runtime URL transformations and requiring
users to provide full URLs in configuration. All providers now use
'base_url' consistently and respect the exact URL provided without
appending paths like /v1 or /openai/v1 at runtime.

BREAKING CHANGE: Users must update configs to include full URL paths
(e.g., http://localhost:11434/v1 instead of http://localhost:11434).

Closes #3732 

## Test Plan

Existing tests should pass even with the URL changes, due to default
URLs being altered.

Add unit test to enforce URL standardization across remote inference
providers (verifies all use 'base_url' field with HttpUrl | None type)

Signed-off-by: Charlie Doern <cdoern@redhat.com>

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

@@ -255,7 +255,7 @@ class WatsonXInferenceAdapter(LiteLLMOpenAIMixin):
         )
 
     def get_base_url(self) -> str:
-        return self.config.url
+        return str(self.config.base_url)
 
     # Copied from OpenAIMixin
     async def check_model_availability(self, model: str) -> bool:
@@ -316,7 +316,7 @@ class WatsonXInferenceAdapter(LiteLLMOpenAIMixin):
         """
         Retrieves foundation model specifications from the watsonx.ai API.
         """
-        url = f"{self.config.url}/ml/v1/foundation_model_specs?version=2023-10-25"
+        url = f"{str(self.config.base_url)}/ml/v1/foundation_model_specs?version=2023-10-25"
         headers = {
             # Note that there is no authorization header.  Listing models does not require authentication.
             "Content-Type": "application/json",