Made session compaction keep complete recent turns and produce clearer summaries for smaller models

Fixes

• CoreAdded MERGE Gateway reasoning variants so those model options work correctly.@MatthewFeroz
• CoreCapped automatic session retries and added jitter to reduce repeated retry storms.
• CoreEnabled PDF attachments for GitHub Copilot models that advertise PDF vision support.@stevenao
• CoreApplied the correct sampling defaults to DeepSeek V4 Flash on supported providers.
• CoreRouted all Muse family models to the correct Meta system prompt.@mreso
• DesktopUpdated Chinese developer terminology to use more widely recognized translations.
• DesktopUsed the server's current default model instead of stale local config when choosing the default model.