fix(provider): strip <think> tags and merge system messages for MiniMax

MiniMax API rejects role: system in the messages array with error
2013 (invalid message role: system). In channel mode, the history
builder prepends a system message and optionally appends a second
one for delivery instructions, causing 400 errors on every channel
turn.

Additionally, MiniMax reasoning models embed chain-of-thought in
the content field as <think>...</think> blocks rather than using
the separate reasoning_content field, causing raw thinking output
to leak into user-visible responses.

Changes:
- Add merge_system_into_user flag to OpenAiCompatibleProvider;
  when set, all system messages are concatenated and prepended to
  the first user message before sending to the API
- Add new_merge_system_into_user() constructor used by MiniMax
- Add strip_think_tags() helper that removes <think>...</think>
  blocks from response content before returning to the caller
- Apply strip_think_tags in effective_content() and
  effective_content_optional() so all non-streaming paths are covered
- Update MiniMax factory registration to use new_merge_system_into_user
- Fix pre-existing rustfmt violation on apply_auth_header call

All other providers continue to use the default path unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/providers/mod.rs

@@ -661,12 +661,14 @@ pub fn create_provider_with_url(
                 AuthStyle::Bearer,
             )))
         }
-        name if minimax_base_url(name).is_some() => Ok(Box::new(OpenAiCompatibleProvider::new(
-            "MiniMax",
-            minimax_base_url(name).expect("checked in guard"),
-            key,
-            AuthStyle::Bearer,
-        ))),
+        name if minimax_base_url(name).is_some() => Ok(Box::new(
+            OpenAiCompatibleProvider::new_merge_system_into_user(
+                "MiniMax",
+                minimax_base_url(name).expect("checked in guard"),
+                key,
+                AuthStyle::Bearer,
+            )
+        )),
         "bedrock" | "aws-bedrock" => Ok(Box::new(OpenAiCompatibleProvider::new(
             "Amazon Bedrock",
             "https://bedrock-runtime.us-east-1.amazonaws.com",