perf: Concretely typed TopK StringHeap, and StringHashTable slot reuse

[MassivePizza]

Which issue does this PR close?

N/A.

Continuation of #23609.

Rationale for this change

Improve perf of TopK.

What changes are included in this PR?

Eliminate overhead of casting with dyn Any for every String in topk::StringHeap.

Rearrange HashTableItem<ID> Option-ness to avoid unnecessary unwraps on lookups in topk::TopKHashTable.
Rewrite inserts to take a borrowable type, delaying clones and allowing reuse of storage slots (mostly Strings).

Are these changes tested?

Should be covered by existing tests.

Making ID an Option<ID> inside HashTableItem should be fine, since the internal HashTable keeps track of valid items (although this could be hard to verify properly). It runs with the assumption that "if everything needed an unwrap, nothing needs an unwrap".

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.49123% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.14%. Comparing base (caa3cb4) to head (ddd9b71).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...on/physical-plan/src/aggregates/topk/hash_table.rs	95.87%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24105   +/-   ##
=======================================
  Coverage   81.14%   81.14%           
=======================================
  Files        1110     1110           
  Lines      386107   386110    +3     
  Branches   386107   386110    +3     
=======================================
+ Hits       313313   313317    +4     
+ Misses      54329    54328    -1     
  Partials    18465    18465           

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