* fix(providers): stop offering rejected API key setup for Mistral and Abacus AI

Remove Mistral and Abacus AI from supportsApiKeySetup in configProviders.qml
because upstream CLI rejects config set-api-key for both. Providers settings
now falls back to provider login, documentation, and dashboard links instead of
presenting a failing Set API key action and an inaccurate Supported status row.

* docs(changelog): clarify Mistral and Abacus AI setup fallback behavior

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Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>