refactor: retain hash join build batches after allocation failure

[peterxcli]

Which issue does this PR close?

Part of #24768. Splits the build refactor out of #25217 and #26015 for easier review.

Rationale for this change

The sort-merge fallback needs the build rows after a hash table allocation fails.

What changes are included in this PR?

Separate hash-table construction from input collection and retain the batches when building or concatenating them fails. Also adds Clippy allowances for Expr hash-set keys.

What is the testing strategy for this PR?

Tests cover retained batches, memory accounting, and existing memory-limit behavior for ordered and unbounded inputs.

Are there any user-facing changes?

None. The fallback comes in a follow-up PR.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.15084% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.78%. Comparing base (4e6e564) to head (18ab171).

Files with missing lines	Patch %	Lines
...tafusion/physical-plan/src/joins/hash_join/exec.rs	87.15%	1 Missing and 22 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #26182    +/-   ##
========================================
  Coverage   82.77%   82.78%            
========================================
  Files        1148     1148            
  Lines      451075   451220   +145     
  Branches   451075   451220   +145     
========================================
+ Hits       373397   373520   +123     
- Misses      54930    54938     +8     
- Partials    22748    22762    +14     

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[peterxcli]

@jayzhan211 ptal, thanks!