fix(database): replace type: ignore with assert for type narrowing in LegacyDataMigrator

Remove the unsafe `# type: ignore` suppression on line 111 of
`legacy_migrator.py` and replace it with an explicit
`assert existing_plan.id is not None` statement. This provides
proper type narrowing to the type checker while preserving the
logical correctness guaranteed by the preceding `if existing_plan:`
guard.

Also adds a new BDD scenario in `legacy_migrator_coverage.feature`
that explicitly exercises the code path where an existing plan with
a non-None id is found during migration, verifying the assert-based
type narrowing works correctly end-to-end.

ISSUES CLOSED: #3051

features/legacy_migrator_coverage.feature

@@ -204,9 +204,17 @@ Feature: Legacy Data Migrator Coverage
     Given I have a legacy project with read-only JSON files
     When I run the legacy data migration
     Then the migration should handle backup errors gracefully
+
   Scenario: get_all_for_project is called only once for multiple plans
     Given I have a legacy project with multiple plans to migrate efficiently
     When I run the legacy data migration with query tracking
     Then get_all_for_project should be called exactly once
     And the migration should return true
     And all plans should be migrated correctly
+
+  Scenario: Map existing plan IDs correctly during migration
+    Given I have a legacy project with plans already in database
+    When I run the legacy data migration
+    Then existing plans should not be duplicated
+    And the migration should return true
+    And the existing plan id should be mapped without type suppression

features/steps/legacy_migrator_steps.py

@@ -1085,3 +1085,18 @@ def step_check_all_plans_migrated(context: Context) -> None:
         assert plan_names == expected_names, (
             f"Expected plans {expected_names}, got {plan_names}"
         )
+
+
+@then("the existing plan id should be mapped without type suppression")
+def step_check_existing_plan_id_mapped(context: Context) -> None:
+    """Check that existing plan id was mapped correctly via assert type narrowing."""
+    with context.unit_of_work.transaction() as ctx:
+        project = ctx.projects.get_by_name("existing_plans")
+        assert project is not None, "Project 'existing_plans' should exist in database"
+        plans = ctx.plans.get_all_for_project(project.id)
+        main_plans = [p for p in plans if p.name == "main"]
+        assert len(main_plans) == 1, "Should have exactly one 'main' plan"
+        assert main_plans[0].id is not None, (
+            "Existing plan id must be non-None (assert narrowing validates this)"
+        )
+

src/cleveragents/infrastructure/database/legacy_migrator.py

@@ -110,7 +110,8 @@ class LegacyDataMigrator:
                             (p for p in all_plans if p.name == plan_name), None
                         )
                         if existing_plan:
-                            plan_id_map[plan_name] = existing_plan.id  # type: ignore
+                            assert existing_plan.id is not None
+                            plan_id_map[plan_name] = existing_plan.id
                             continue
 
                         # Create new plan