planner: align core casetest explain outputs on plan_tree

## Enhancement

`pkg/planner/core/casetest` still mixes default `explain`, `explain format = 'brief'`, and `explain format = 'plan_tree'` in ordinary planner assertions.

That inconsistency makes the recorded outputs harder to maintain and leaves inline assertions coupled to brief-only columns such as `estRows` and generated `Column#...` names.

The cleanup should:

- migrate ordinary planner core casetests to `explain format = 'plan_tree'`
- keep existing special cases unchanged:
- explain-format-specific tests
- `explain for connection`
- plan cache tests
- cost trace tests
- TPCH / TPCDS tests
- `explain analyze` cases that do not support `plan_tree`
- refresh the affected golden outputs and inline expectations

Expected result:

- non-specialized planner core casetests use one explain output shape
- the recorded outputs stay consistent across packages
- inline assertions no longer depend on brief-only columns where `plan_tree` is used