Fix decomposed pipeline phase 4 rollout

What does this MR do and why?

In !76188 (merged) we introduced a new feature flag force_no_sharing_primary_model which broke a whole bunch of specs in the ~"pipeline:run-decomposed" pipeline which is optional. Since I never ran that pipeline I never noticed this.

This fixes the problem by globally disabling this feature flag in specs by default as it doesn't make sense to be enabled by default. Eventually in !78182 (merged) we'll be merging changes so that running fully decomposed with 2 write connections will be the default but for now there are too many problems to fix that we don't need to change alongside this work.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.