fix: resolve remaining mypy type errors in COPT solver

Fix type compatibility issues in solvers.py:
- Convert condition to string for Status.from_termination_condition()
- Ensure legacy_status assignment matches expected str type

Resolves the final mypy errors, completing issue #367.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: FabianHofmann

linopy/solvers.py

@@ -2041,9 +2041,9 @@ def solve_problem_from_file(
 
         # TODO: check if this suffices
         condition = m.MipStatus if m.ismip else m.LpStatus
-        termination_condition = CONDITION_MAP.get(condition, condition)
+        termination_condition = CONDITION_MAP.get(condition, str(condition))
         status = Status.from_termination_condition(termination_condition)
-        status.legacy_status = condition
+        status.legacy_status = str(condition)
 
         def get_solver_solution() -> Solution:
             # TODO: check if this suffices