Enable prefetchInlining by default (#92863)
Flips the `experimental.prefetchInlining` default from `false` to
`true`. This is the first step toward removing the flag entirely.
Tests that explicitly set `prefetchInlining: true` have been cleaned up
since they now match the default. Any CI failures from this PR will be
addressed in follow-up PRs by temporarily opting out individual tests
with `prefetchInlining: false` until they are migrated.
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