minor(test): strengthen sort-merge join spilling coverage

[buraksenn]

Which issue does this PR close?

• Part of Improve test coverage for spilling (memory-limited) sort/aggregation/sort-merge-join #13431

Rationale for this change

Contuining the work after #23947. Now SortMergeJoinExec supports spilling under memory pressure, but existing tests do not consistently assert that spilling occurs.

What changes are included in this PR?

Adds spill fuzz coverage, constrained-memory stress tests for materializing joins, and process-isolated memory-limit validation.

Are these changes tested?

test only change and it can be tested via:

cargo test -p datafusion --test fuzz --features extended_tests spill
cargo test -p datafusion --test core_integration --features extended_tests smj_

Are there any user-facing changes?

no test only change

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.75%. Comparing base (994fc81) to head (f45a5cd).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23988      +/-   ##
==========================================
+ Coverage   80.69%   80.75%   +0.05%     
==========================================
  Files        1095     1096       +1     
  Lines      372626   373588     +962     
  Branches   372626   373588     +962     
==========================================
+ Hits       300700   301687     +987     
+ Misses      53978    53897      -81     
- Partials    17948    18004      +56     

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