fix(qwen): support reasoning-model params (max_completion_tokens, omit temperature) (#128)

The qwen_chat backend (the generic OpenAI-compatible client) hardcoded
max_tokens and always sent temperature, so reasoning models behind
OpenAI-compatible gateways (GPT-5.x, Claude Opus 4.8 via Azure/LiteLLM)
would 400.

- Add opt-in QWEN_CHAT_USE_MAX_COMPLETION_TOKENS (+ role variants) that
  swaps the payload key max_tokens -> max_completion_tokens.
- Treat an explicit empty / none / off temperature as "omit" instead of
  collapsing to the 0.7 default (via _resolve_temperature).
- Thread both through configure_qwen_chat / _update_config.
- Defaults unchanged; fully backward compatible. Adds 6 tests.

Fixes #127

Co-authored-by: Chirag Singhal <chirag127@users.noreply.github.com>

skillopt/model/__init__.py

@@ -13,13 +13,13 @@ from skillopt.model.backend_config import (  # noqa: F401
     configure_codex_exec,
     get_claude_code_exec_config,
     get_codex_exec_config,
-    get_target_backend,
     get_optimizer_backend,
+    get_target_backend,
+    is_optimizer_chat_backend,
     is_target_chat_backend,
     is_target_exec_backend,
-    is_optimizer_chat_backend,
-    set_target_backend,
     set_optimizer_backend,
+    set_target_backend,
 )
 
 
@@ -440,18 +440,21 @@ def configure_qwen_chat(
     timeout_seconds: float | str | None = None,
     max_tokens: int | str | None = None,
     enable_thinking: bool | str | None = None,
+    use_max_completion_tokens: bool | str | None = None,
     optimizer_base_url: str | None = None,
     optimizer_api_key: str | None = None,
     optimizer_temperature: float | str | None = None,
     optimizer_timeout_seconds: float | str | None = None,
     optimizer_max_tokens: int | str | None = None,
     optimizer_enable_thinking: bool | str | None = None,
+    optimizer_use_max_completion_tokens: bool | str | None = None,
     target_base_url: str | None = None,
     target_api_key: str | None = None,
     target_temperature: float | str | None = None,
     target_timeout_seconds: float | str | None = None,
     target_max_tokens: int | str | None = None,
     target_enable_thinking: bool | str | None = None,
+    target_use_max_completion_tokens: bool | str | None = None,
 ) -> None:
     _qwen.configure_qwen_chat(
         base_url=base_url,
@@ -460,18 +463,21 @@ def configure_qwen_chat(
         timeout_seconds=timeout_seconds,
         max_tokens=max_tokens,
         enable_thinking=enable_thinking,
+        use_max_completion_tokens=use_max_completion_tokens,
         optimizer_base_url=optimizer_base_url,
         optimizer_api_key=optimizer_api_key,
         optimizer_temperature=optimizer_temperature,
         optimizer_timeout_seconds=optimizer_timeout_seconds,
         optimizer_max_tokens=optimizer_max_tokens,
         optimizer_enable_thinking=optimizer_enable_thinking,
+        optimizer_use_max_completion_tokens=optimizer_use_max_completion_tokens,
         target_base_url=target_base_url,
         target_api_key=target_api_key,
         target_temperature=target_temperature,
         target_timeout_seconds=target_timeout_seconds,
         target_max_tokens=target_max_tokens,
         target_enable_thinking=target_enable_thinking,
+        target_use_max_completion_tokens=target_use_max_completion_tokens,
     )