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

[adriangb]

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. #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.

Which issue does this PR close?

• Closes #.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[adriangb]

@andygrove a refactor related to the proto tests

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.98%. Comparing base (f4c8ba1) to head (d792cc5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24223      +/-   ##
==========================================
- Coverage   80.98%   80.98%   -0.01%     
==========================================
  Files        1106     1106              
  Lines      383232   383232              
  Branches   383232   383232              
==========================================
- Hits       310372   310371       -1     
+ Misses      54543    54540       -3     
- Partials    18317    18321       +4     

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[alamb]

This is a nice cleanup -- thank you @adriangb

I had claude review and confirm that this is a pure code motion refactor

Give its high liklihood of conflicts, I will put it in the merge queue immediately

[adriangb]

Thanks so much @alamb !