Summary

Fixes #5719. Destructive schema changes on forward-only models are now surfaced at plan-build time during dev plans (sqlmesh plan dev), not just prod plans.

Problem

When a forward-only model has columns removed or types narrowed, sqlmesh plan dev and PR checks would succeed, but the production deployment would fail with a DestructiveChangeError during MigrateSchemasStage. This left the production environment in an unfinalized state.

Root Cause

PlanBuilder._check_destructive_additive_changes already checked for destructive schema changes at plan-build time (regardless of is_dev), but only for directly modified snapshots. Indirectly modified forward-only models (e.g., downstream models inheriting schema changes from an upstream dependency) were not checked. The evaluator-level check in MigrateSchemasStage catches these for prod, but that stage is skipped for dev plans.

Changes

sqlmesh/core/plan/builder.py

• Extract schema diffing into reusable _check_schema_change helper method
• Pass indirectly_modified mapping to _check_destructive_additive_changes
• Add a second loop to check indirectly modified forward-only snapshots for destructive/additive schema changes

tests/core/test_plan.py

Add 5 new tests for is_dev=True scenarios:

• test_forward_only_destructive_change_dev_plan — ERROR mode raises PlanError
• test_forward_only_destructive_change_dev_plan_warn — WARN mode logs warning
• test_forward_only_destructive_change_dev_plan_allow — ALLOW mode passes silently
• test_forward_only_indirect_destructive_change_dev_plan — indirect modification path exercised
• test_forward_only_flag_destructive_change_dev_plan — --forward-only flag with dev plan

Test Results

All 27 forward-only/destructive tests pass (5 new + 22 existing), zero regressions.