Fix ensemble contour map GENERATED result statistics and caching bugs

[magnesj]

Summary

Fixes #14812 - a set of related bugs affecting ensemble contour maps that use a GENERATED (grid calculation) result, plus a related bug where switching a view's case lost generated results.

1. Per-realization recompute: RimStatisticsContourMap::computeStatisticsForMaps() opens and closes each ensemble realization in turn to bound memory use. RimEclipseCase::closeReservoirCase() discards the whole RigEclipseCaseData, so a GENERATED result computed for one realization is lost again before the next realization is processed. Added ensureGeneratedResultIsComputed(), called for the current case inside the per-case loop, which looks up the matching RimGridCalculation (by result variable name) and re-runs it for that single realization.

2. Disk cache always rejected for GENERATED results: RimStatisticsContourMap::selectedTimeSteps() collapses to a single time step unless the mapped result reports as dynamic (RimEclipseResultDefinition::hasDynamicResult()). For a GENERATED result, that check depends on whether the primary case currently has the result computed in memory - true right after computing statistics (cache write time), false on a fresh session before the calculation has run again (cache check time). This made the cache validity key unstable between sessions, so the on-disk .gri cache was always rejected and the whole ensemble recomputed on every load. Fixed by proactively computing the GENERATED result on the primary case in ensureResultsComputed(), before the cache is checked.

3. "Calculate" button always recomputed the whole ensemble: clicking "Calculate" for a calculation with a Destination Ensemble iterated every realization, even for a plain per-cell expression whose result can be recomputed lazily, one realization at a time (see point 1). Restricted RimGridCalculation::calculate() to the ensemble's main case for plain expressions via a new casesToCalculate() helper; only aggregation expressions (sum/avg/min/max/count, whose result is the per-realization summary itself and cannot be produced lazily) still compute for every realization.

4. Generated results lost when a view is stepped to another case: grid calculation results are not stored with the case, so switching a view's case (RimEclipseView::fieldChangedByUi()) left no generated results available and the view was cleared. Added RimGridCalculationCollection::ensureCalculationsAreComputed(), called before the result definitions are updated, which computes any calculation missing for the new case. Introduced RimGridCalculation::calculateForCase() to compute a single case using the calculation's own filter and time step settings, and reused it in ensureGeneratedResultIsComputed() for the ensemble contour map per-realization path (point 1), so that path now also respects the calculation's filter/time step settings instead of always using none.

Testing

• Added RimStatisticsContourMapTest.GeneratedDynamicResultIsNotReportedAsDynamicBeforeItIsComputed and RimStatisticsContourMapTest.EnsureResultsComputedMakesGeneratedResultDynamicBeforeCacheCheck reproducing and verifying the caching fix.
• Added RimGridCalculationTest.EnsembleDestinationWithPlainExpressionOnlyCalculatesMainCase and RimGridCalculationTest.EnsembleDestinationWithAggregationExpressionCalculatesAllCases verifying the "Calculate" button restriction.
• Added RimGridCalculationTest.EnsureCalculationsAreComputedForCaseNotInOutputCases verifying generated results are computed for a case stepped to that is not an output case of the calculation.

[kriben]

Reviewed the last commit using open-code-review-delegate.

[magnesj]

Addressed both review comments in 21d609e:

• High (open source cases before validation): calculateForCase() now opens any closed source cases before validation/evaluation, mirroring what calculate() already did. Added RimGridCalculationTest.CalculateForCaseOpensClosedSourceCase reproducing the crash (closed source case -> null results() deref) and verifying it's fixed.
• Medium (invalidate stale lazily-computed results): when calculate() restricts a plain per-cell ensemble expression to the main case, it now clears the output result on any other already-open realization via RigCaseCellResultsData::clearScalarResult(), and RimGridCalculationCollection::ensureCalculationsAreComputed() now checks isResultLoaded() in addition to hasResultEntry() so a cleared-but-registered result is recomputed on next use instead of being treated as up to date. Added RimGridCalculationTest.RecalculatingEnsembleMainCaseInvalidatesStaleResultOnOtherOpenRealization covering this.

All 44 tests in the targeted suite pass, and the full ResInsight app target builds cleanly.