Fix uninitialized memory in allocate_daqp_workspace and Eigen interface

[Copilot]

Valgrind identified 111 uninitialized-memory errors across the Eigen C++ tests (03, 04, 05) and zero memory leaks. Three bugs fixed:

• work->m / work->ms never initialized (src/api.c): allocate_daqp_workspace set work->n but left work->m and work->ms as garbage. The DAQP class calls this function in its constructor, and resize_result then compares m != work_.m before daqp_update_ldp has had a chance to populate those fields.

  // Before
  work->n = n;
  n = n + ns;
  
  // After
  work->n = n;
  work->m = 0;
  work->ms = 0;
  n = n + ns;

• slack_ allocated but not zeroed (daqp.cpp): EigenDAQPResult::resize_dual used slack_.resize(m), which allocates without initializing. Callers copying the result and calling get_slack() before set_slack() received garbage values. Changed to slack_.setZero(m).

• DAQP::solve(A, bu, bl, break_points) returned result with unpopulated slack (daqp.cpp): Unlike the standalone daqp_solve(), the class method never called set_slack before returning. Added a get_slack() call so that a copied EigenDAQPResult carries valid slack data, matching the static function's behavior.

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