two bugs. both squash user model choice silently.
bug 1: applyDefaultModel() unconditional primary: model overwrite.
wizard calls with setDefaultModel=true, provider returns its default
(e.g. openrouter/auto), bam user primary gone. fix: existingPrimary ?? model.
bug 2: applyModelFallbacksFromSelection() phantom primary injection.
when no primary configured, resolvedKey (hardcoded default) written as
primary via nullish coalescing fallback. fix: conditional spread — only
include primary key when one actually existed.
tests for both. closes#70696
Sathvik-1007
8d57d745cf