test(proto): split roundtrip_physical_plan.rs by plan category - #24223

…e#24223)

`roundtrip_physical_plan.rs` had grown to 5,682 lines, so finding the
tests for a given plan — or noticing that a plan has none — meant
scrolling through every other plan's tests. apache#24171 (e) proposed
splitting it by plan category; this does that.

The tests now live in `tests/cases/plans/`, grouped by what they cover:
leaves, dispatch, limits, filters, joins, sorts, aggregates, windows,
sources, sinks, udfs, exprs, dynamic_filters, scalar_subquery, misc and
tpch. The shared round-trip helpers (`roundtrip_test` and friends) stay
in `plans/mod.rs`; every other item moved into exactly one file, and
each file carries its own imports rather than inheriting one 140-line
block.

Pure code motion: no test was added, removed, renamed or edited. Every
moved item is byte-identical to its previous form, and the test
inventory (`--list`) is unchanged at 222 entries.

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Co-authored-by: Claude <[email protected]>