Refill pairing teammate pool after partial acceptance

[kylehoehns]

Summary

• When a teammate accepts one or more slots but the session still needs more people, we now bring in the next person in the queue — mirroring how the decline path works
• Previously, an acceptance shrank the pending pool permanently (e.g. 5 invited → 1 partial accept → stuck at 4 pending)
• After recordSlotSelections + closeIfComplete, if the session is still active we call requestNextTeammate to top the pool back up

Test plan

• pnpm verify passes (new tests cover both the refill and the closed-session no-op)
• Manually accept a partial slot on a pairing session and confirm a new teammate is DM'd