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fix(mongodb-runner): fix precedence bug in replset primary selection (#950)

`m.priority ?? 0 >= (m2.priority ?? 0)` parses as
`m.priority ?? (0 >= ...)`, so any member with an explicitly set
priority was treated as highest regardless of its value. Both call
sites (the member lookup in processRSMembers and the primary
computation in _startServers) now use a shared findPrimary() helper
that returns the primary member with the intended comparison.

On a priority tie, findPrimary() prefers a member with an unset
priority, since replSetInitiate defaults the initial primary's unset
priority to 1. Without this rule, `rsMembers: [{ priority: 0 }, {}]`
selected the explicitly-zero member, every member ended up with
priority 0, and the server rejected the configuration ("must contain
at least one non-arbiter member with priority > 0" — verified against
mongod 8.0, which now yields priorities [0, 1] with the unset member
as primary). Unit tests cover highest-priority selection, ties,
unset-vs-explicit priorities, and an empty member list.

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