[ML] Relax Bayesian optimisation mean improvement threshold for debug builds

The testMaximumExpectedImprovement test requires BO mean improvement to
exceed 1.5x random search, but with the reduced iteration count in debug
builds (15 vs 30) BO doesn't accumulate enough advantage to consistently
hit that bar. This has been failing deterministically on macOS and Windows
in every nightly debug build.

Introduce a MEAN_IMPROVEMENT_MULTIPLIER constant (1.5 for release, 1.2
for debug) alongside the existing debug-mode relaxations for NUM_TRIALS,
NUM_BO_ITERATIONS, and WIN_RATE_THRESHOLD.

Made-with: Cursor

Author: edsavage

lib/maths/common/unittest/CBayesianOptimisationTest.cc

@@ -286,13 +286,17 @@ BOOST_AUTO_TEST_CASE(testMaximumExpectedImprovement) {
     constexpr std::size_t NUM_TRIALS{50};
     constexpr std::size_t NUM_BO_ITERATIONS{30};
     constexpr double WIN_RATE_THRESHOLD{0.95};
+    constexpr double MEAN_IMPROVEMENT_MULTIPLIER{1.5};
 #else
     // Unoptimised Eigen makes each maximumExpectedImprovement() call ~100x
     // slower.  Reduce the workload so the test completes in a few minutes
-    // rather than 90+ in debug builds.
+    // rather than 90+ in debug builds.  With fewer iterations the BO
+    // advantage over random search is less pronounced so we also relax
+    // the mean improvement multiplier.
     constexpr std::size_t NUM_TRIALS{10};
     constexpr std::size_t NUM_BO_ITERATIONS{15};
     constexpr double WIN_RATE_THRESHOLD{0.7};
+    constexpr double MEAN_IMPROVEMENT_MULTIPLIER{1.2};
 #endif
 
     test::CRandomNumbers rng;
@@ -384,7 +388,8 @@ BOOST_AUTO_TEST_CASE(testMaximumExpectedImprovement) {
               << 100.0 * maths::common::CBasicStatistics::mean(meanImprovementRs));
     BOOST_TEST_REQUIRE(wins > static_cast<std::size_t>(WIN_RATE_THRESHOLD * NUM_TRIALS));
     BOOST_TEST_REQUIRE(maths::common::CBasicStatistics::mean(meanImprovementBopt) >
-                       1.5 * maths::common::CBasicStatistics::mean(meanImprovementRs));
+                       MEAN_IMPROVEMENT_MULTIPLIER *
+                           maths::common::CBasicStatistics::mean(meanImprovementRs));
 }
 
 BOOST_AUTO_TEST_CASE(testKernelInvariants) {