Use the registry's default_model() accessor so inline assist can use the
same automatically selected fallback model as agent sessions when no
configured default model resolves. Keep explicit inline model selection
as the first choice.

Related to #59309.

Release Notes:

- Fixed Inline Assistant failing to select an available fallback model
when no default model is configured.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>