Validate GMM simulate params at request boundary

Add defensive validation of the GMM parameters in the FX-spot simulate path so malformed input is rejected with a clear error rather than risking a fatal abort or silently-degenerate output. The simulate handler (projects/ores.synthetic/service/src/simulate_handler.hpp) already wraps simulation in a try/catch and checks gmm_means is non-empty, but it does not validate the per-component values. Validate at the request boundary that: gmm_means, gmm_stdevs and gmm_weights are all the same length; every stdev is finite and > 0; every weight is finite and > 0 with a positive sum; means are finite; and initial_price > 0. On failure, reply with success = false and a descriptive message instead of proceeding. Consider mirroring the cheap finite/range checks on the client (SamplePricePathsChart) so the UI can flag bad config before sending.

The σ = 0 ("Flat" profile) crash showed that bad GMM parameters could reach std::normal_distribution and trip a libstdc++ assertion, aborting the synthetic service (SIGABRT) and surfacing to the user only as an opaque NATS timeout. The immediate crash is now fixed in the process implementations (degenerate σ <= 0 draws the mean), but a try/catch cannot catch an abort(), so the handler should not rely on the process layer being the only guard. Validating inputs at the boundary turns any future bad-parameter case (NaN/Inf, negative stdev, zero/negative weight sum, mismatched lengths) into a clean, user-visible error and a more robust service.

• projects/ores.synthetic/service/src/simulate_handler.hpp — request handler; current checks only that gmm_means is non-empty.
• projects/ores.synthetic/service/src/processes/gmm_process.cpp — now guards σ <= 0 (draws the mean) after the SIGABRT fix.
• projects/ores.synthetic/service/src/processes/arithmetic_gmm_process.cpp — same guard applied.
• projects/ores.qt/synthetic/src/SamplePricePathsChart.cpp — client that builds the simulate request.