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.