Fail fast when a myopic stage has no solution

[mohammadrezwankhan]

Description Fixes the myopic multistage workflow so a failed stage is reported at the point of failure. solve_model can return a model without primal values after logging that no solution was found. The myopic loop previously continued, and the next stage called fix_initial_investments on the unsolved prior model. That masked the original solver failure with a downstream ResultIndexBoundsError from value(...). This change checks has_values immediately after each myopic stage solve and aborts with the stage number, termination status, primal status, and raw solver status. Successful-stage behavior is unchanged. ## What type of PR is this? (check all applicable) - [ ] Feature - [x] Bug Fix - [ ] Documentation Update - [ ] Code Refactor - [ ] Performance Improvements ## Related Tickets & Documents Related to Discussion #917, where the downstream exception obscured a Stage 1 solve failure. ## Checklist - [x] Code changes are sufficiently documented; no user-facing configuration or API changed. - [x] The change is based on the latest main commit. - [x] Code has been tested to ensure all functionality works as intended. - [x] CHANGELOG.md includes an Unreleased > Fixed entry for #918. - [x] I consent to the release of this PR's code under the GNU General Public license. ## How this can be tested Tested with Julia 1.12.6 and HiGHS: - Added a two-stage end-to-end myopic-run test whose first stage is infeasible; it verifies the runner reports Stage 1 directly instead of entering the Stage 2 linking logic. - Verified the message contains termination_status=INFEASIBLE, primal_status=NO_SOLUTION, and the raw solver status. - Ran test/test_multistage.jl successfully. The focused test reports: text Myopic run stops at a stage without a solution | 5 passed

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