Stop exposing litellm/ prefix in user-facing model names

Users had to type STRIX_LLM=litellm/deepseek/deepseek-chat — the
litellm/ wrapper was Strix-internal plumbing surfacing in user config.

Add StrixProvider, a MultiProvider subclass that routes any non-OpenAI
prefix (deepseek/, anthropic/, groq/, xai/, mistral/, openrouter/, …)
through LitellmProvider with the prefix preserved. normalize_model_name
no longer adds litellm/ to anything; bare claude-* / gemini-* shorthands
expand to anthropic/<model> / gemini/<model> instead of the wrapped form.

Wire StrixProvider into warm_up_llm and RunConfig.model_provider.
litellm/<provider>/<model> and any-llm/<provider>/<model> still resolve
unchanged for users on older config.

Refresh stale model names in the env-validation messages and the
warm-up hint (gpt-5.4, claude-opus-4-7, deepseek-reasoner).

Verified 24-case end-to-end matrix: OpenAI direct vs. LitellmProvider
routing, env-var mirroring via validate_environment, supports_reasoning
detection, and tool_choice gating all behave correctly across modern
providers including the user's unknown DeepSeek SKU.

strix/core/runner.py

@@ -15,6 +15,7 @@ from agents.sandbox import SandboxRunConfig
 from strix.agents.factory import build_strix_agent, make_child_factory
 from strix.config import load_settings
 from strix.config.models import (
+    StrixProvider,
     configure_sdk_model_defaults,
     normalize_model_name,
     uses_chat_completions_tool_schema,
@@ -159,6 +160,7 @@ async def run_strix_scan(
         )
         run_config = RunConfig(
             model=resolved_model,
+            model_provider=StrixProvider(),
             model_settings=model_settings,
             sandbox=SandboxRunConfig(client=bundle["client"], session=bundle["session"]),
             trace_include_sensitive_data=False,