fix(model): route chat_optimizer through minimax_chat (not just chat_target) (#116)

Two missing pieces in the minimax_chat dispatch chain:

1. minimax_backend.py did not define chat_optimizer or chat_optimizer_messages;
   only chat_target / chat_target_messages existed. Any caller using
   optimizer_backend='minimax_chat' would hit AttributeError or fall through
   to _openai (Azure).

2. skillopt/model/__init__.py's chat_optimizer and chat_optimizer_messages
   dispatchers checked claude_chat and qwen_chat but not minimax_chat,
   so minimax_chat callers would silently fall through to _openai.chat_optimizer
   (the Azure path), which fails with 'Azure OpenAI endpoint is not configured'
   on any setup without AZURE_OPENAI_* env vars.

Adds chat_optimizer to minimax_backend.py (mirrors chat_target via
_chat_messages_impl) and minimax_chat branches to both
chat_optimizer / chat_optimizer_messages dispatchers.

Verified locally: 1-epoch training on a 4-item SearchQA-format dataset
went from '[skip] no usable patches — skill unchanged' (baseline fallback)
to a successful accept_new_best with success_patches=1 per step.

Co-authored-by: Mavis (MiniMax) <Mavis@MiniMax.local>
Co-authored-by: jc <jc@users.noreply.github.com>

skillopt/model/__init__.py

@@ -105,6 +105,16 @@ def chat_optimizer(
             reasoning_effort=reasoning_effort,
             timeout=timeout,
         )
+    if get_optimizer_backend() == "minimax_chat":
+        return _minimax.chat_optimizer(
+            system=system,
+            user=user,
+            max_completion_tokens=max_completion_tokens,
+            retries=retries,
+            stage=stage,
+            reasoning_effort=reasoning_effort,
+            timeout=timeout,
+        )
     return _openai.chat_optimizer(
         system=system,
         user=user,
@@ -204,6 +214,18 @@ def chat_optimizer_messages(
             return_message=return_message,
             timeout=timeout,
         )
+    if get_optimizer_backend() == "minimax_chat":
+        return _minimax.chat_target_messages(
+            messages=messages,
+            max_completion_tokens=max_completion_tokens,
+            retries=retries,
+            stage=stage,
+            reasoning_effort=reasoning_effort,
+            tools=tools,
+            tool_choice=tool_choice,
+            return_message=return_message,
+            timeout=timeout,
+        )
     return _openai.chat_optimizer_messages(
         messages=messages,
         max_completion_tokens=max_completion_tokens,