fix: set authHeader: true by default for MiniMax API provider (#27622)

* Update onboard-auth.config-minimax.ts

fix issue #27600

* fix(minimax): default authHeader for implicit + onboarding providers (#27600)

Landed from contributor PR #27622 by @riccoyuanft and PR #27631 by @kevinWangSheng.
Includes a small TS nullability guard in lane delivery to keep build green on rebased head.

Co-authored-by: riccoyuanft <riccoyuan@gmail.com>
Co-authored-by: Kevin Shenghui <shenghuikevin@github.com>

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>
Co-authored-by: Kevin Shenghui <shenghuikevin@github.com>

src/commands/onboard-auth.config-minimax.ts

@@ -181,6 +181,7 @@ function applyMinimaxApiProviderConfigWithBaseUrl(
     ...existingProviderRest,
     baseUrl: params.baseUrl,
     api: "anthropic-messages",
+    authHeader: true,
     ...(normalizedApiKey?.trim() ? { apiKey: normalizedApiKey } : {}),
     models: mergedModels.length > 0 ? mergedModels : [apiModel],
   };