Goal

Finish queryPlanner.rankerChoice after SERVER-132229 (chosenRanker) and SERVER-132377 (reason) land. This is the finishing sub-task of SERVER-132104: when it closes, the parent closes.

Work items

1. Populate requestedRanker — the ranker the internal logic understood to be requested, resolved from the dispatch inputs (internalQueryPlanRanker, internalQueryMixedPlanRankingStrategy, featureFlagCostBasedRanker): one of multiPlanning, costBased, Mixed/NoMultiplanningResults, Mixed/EstimateRankingEffort, none.
2. Remove all remaining empty-string placeholders from the rankerChoice emission and discharge the corresponding TODO markers. Rationale: the final output must honor the design principle "absent, never empty" — a value that was not computed is omitted, not emitted as "".
3. Add the exhaustive validity-matrix test: assert that only valid (requestedRanker, chosenRanker, reason) triples per the design doc's combination table can be produced, across the planRanker x mixedPlanRankingStrategy configuration space. (Representative per-field tests land with SERVER-132229/SERVER-132377; this matrix asserts the combined object and therefore lives here.)
4. Record an explicit decision on the optional phases decision trail (the design doc's may-defer enrichment): defer to a new ticket or drop with rationale, so it does not silently evaporate at parent closure.

Dependencies

Depends on SERVER-132229 and SERVER-132377 — items 2 and 3 cannot complete before reason is real.