fix(plan): add explainer visit method for PhysicalLayerSchemaCreationStage

[nkwork9999]

Fixes #5619

Problem

RichExplainerConsole has no visit_physical_layer_schema_creation_stage, so when a plan contains PhysicalLayerSchemaCreationStage, explain() falls into the hasattr guard at explainer.py:134:

sqlmesh.core.plan.explainer - ERROR - Unexpected stage: PhysicalLayerSchemaCreationStage

The stage is then silently dropped from the explained plan, even though PlanEvaluator handles it in evaluator.py:209.

Change

Add the missing visit method. It mirrors the schema derivation in SnapshotEvaluator.create_physical_schemas / _create_schemas — non-symbolic models only, table name resolved through the stage's deployability index, deduplicated by (db, catalog) — and lists the distinct physical schemas that will be created. Returns None when there is nothing to create, consistent with the other Optional[Tree] visit methods.

Placed directly before visit_physical_layer_update_stage to match the stage ordering produced in stages.py.

Output on examples/sushi:

├── Create physical schemas if they do not exist
│   └── memory.sqlmesh__sushi
├── Backfill models by running their queries and run standalone audits
│   ├── sushi.waiter_revenue_by_day -> ...

Tests

test_plan_explain previously only asserted that explaining does not raise, which is why this went unnoticed. It now asserts that no stage is reported as unexpected, which covers this stage and guards against the same omission for stages added later.

Verified the test fails on main with the fix reverted:

E  AssertionError: assert 'Unexpected stage' not in '...'
E    'Unexpected stage' is contained here:
E      er.py:134 Unexpected stage: PhysicalLayerSchemaCreationStage

tests/core/test_plan.py, tests/core/test_plan_stages.py, tests/core/test_context.py and tests/core/integration/test_plan_options.py pass (207 tests), as do ruff format, ruff check and mypy on the changed files.

[StuffbyYuki]

@nkwork9999 Thanks for this pr!

No blocker, but do you think it's worth asserting the explained output includes the schema-creation node (e.g. “Create physical schemas…” / sqlmesh__sushi), not only that “Unexpected stage” is absent. Today’s guard catches missing handlers but not wrong/empty rendering.

[StuffbyYuki]

@nkwork9999 Just wanted to follow up on this!

[StuffbyYuki]

@nkwork9999 Bump

[nkwork9999]

Thanks for following up, @StuffbyYuki. I have pushed the requested test update in 2841c99. test_plan_explain now captures the rendered explanation and asserts both “Create physical schemas if they do not exist” and memory.sqlmesh__sushi, while retaining the existing unexpected-stage and unapplied-plan checks.

I also verified that replacing the schema-creation visitor with an empty renderer makes the new output assertion fail, even without an unexpected-stage error. The related plan, plan-stage, context, and plan-options suites pass: 214 tests. Ruff lint/format and the migration-number check pass. The full make style run still reports mypy errors from missing optional development dependencies in my local environment; I am not claiming that whole check passed.