Strip model-aware branches from LLM configuration

Drop every hand-rolled provider table and per-model gating that had
accumulated in the model-handling layer:

  * normalize_model_name no longer auto-prefixes bare claude-* / gemini-*
    names. Users supply the full <provider>/<model> form. The function
    became literally model_name.strip(), so callers now inline that and
    the function is removed.
  * tool_choice="required" is gone everywhere. Thinking-mode endpoints
    (Anthropic, DeepSeek /beta) reject it; modern reasoning models don't
    need it; non-interactive runs already have
    _append_noninteractive_tool_required_message as the convergence
    backstop. model_supports_reasoning, model_known_to_registry, and
    _model_cost_entry were only used to gate this and follow it out.
  * Reasoning(effort=...) is now attached whenever
    STRIX_REASONING_EFFORT is non-none. litellm.drop_params=True absorbs
    it for non-reasoning models.
  * Warm-up's bare-name OpenAI 401 hint is removed (false-positive prone,
    relied on substring matching).
  * reset_tool_choice on SandboxAgent is no-op now (no tool_choice gets
    set) and is removed.
  * report/dedupe.py was still routing through stock MultiProvider, so
    non-OpenAI configs failed the dedupe LLM pass; switch it to
    StrixProvider.

Verified end-to-end against modern provider strings (openai/gpt-5.4,
anthropic/claude-opus-4-7, deepseek/deepseek-reasoner,
gemini/gemini-2.5-pro, groq/, xai/, mistral/, together_ai/, perplexity/,
openrouter/, litellm/ legacy form, and whitespace-padded input): 18/18
cases route correctly, env vars mirror via litellm.validate_environment,
and ModelSettings carries no tool_choice. mypy strict passes.

strix/agents/factory.py

@@ -395,7 +395,6 @@ def build_strix_agent(
         instructions=instructions,
         tools=tools,
         tool_use_behavior=_finish_tool_use_behavior,
-        reset_tool_choice=interactive,
         model=None,
         capabilities=[
             Filesystem(