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/vllm/config.py

@@ -6,7 +6,7 @@
 
 from pathlib import Path
 
-from pydantic import Field, SecretStr, field_validator
+from pydantic import Field, HttpUrl, SecretStr, field_validator
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack_api import json_schema_type
@@ -14,7 +14,7 @@ from llama_stack_api import json_schema_type
 
 @json_schema_type
 class VLLMInferenceAdapterConfig(RemoteInferenceProviderConfig):
-    url: str | None = Field(
+    base_url: HttpUrl | None = Field(
         default=None,
         description="The URL for the vLLM model serving endpoint",
     )
@@ -48,11 +48,11 @@ class VLLMInferenceAdapterConfig(RemoteInferenceProviderConfig):
     @classmethod
     def sample_run_config(
         cls,
-        url: str = "${env.VLLM_URL:=}",
+        base_url: str = "${env.VLLM_URL:=}",
         **kwargs,
     ):
         return {
-            "url": url,
+            "base_url": base_url,
             "max_tokens": "${env.VLLM_MAX_TOKENS:=4096}",
             "api_token": "${env.VLLM_API_TOKEN:=fake}",
             "tls_verify": "${env.VLLM_TLS_VERIFY:=true}",