Fix retained-capacity accounting for grouped byte MIN/MAX

[kosiew]

Which issue does this PR close?

• Part of feat: analyze memory usage for size functions #23393

Rationale for this change

Grouped byte MIN/MAX accumulators can underreport memory usage because their accounting is based on the current data length and number of group slots rather than the memory allocations they retain.

For example, replacing a 128-byte string with a one-byte string reuses the existing vector allocation, but the previous accounting reports only the new string length. Similarly, unused capacity in the outer group vector is not counted.

This change makes memory accounting reflect retained allocations, providing more accurate memory usage estimates for grouped MIN/MAX operations.

What changes are included in this PR?

• Add total_data_capacity to MinMaxBytesState to track retained inner Vec<u8> capacities independently of total_data_bytes, which remains responsible for output preallocation.
• Update set_value() to maintain capacity accounting when values are inserted, replaced, or cause an existing vector to grow.
• Update size() to account for the outer min_max vector's capacity and the cached inner vector capacities, without double-counting the inline Option<Vec<u8>> descriptors.
• Keep size() O(1) by maintaining the inner capacity total incrementally.
• Update emit_to() to reset capacity accounting on full emission and subtract emitted inner vector capacities on partial emission.

Are these changes tested?

Three regression tests are added:

• size_counts_outer_capacity: Verifies that memory accounting includes allocated outer vector capacity, even when fewer group slots are initialized.
• size_retains_inner_capacity: Tests both MIN and MAX accumulators, verifying that replacing a long string with a shorter one preserves retained-capacity accounting and that partial and full emission update it correctly.
• size_tracks_growth_and_emit_reuse: Verifies capacity accounting when an inner vector grows, groups are partially emitted, and the state is reused after full emission.

Are there any user-facing changes?

No changes to SQL MIN/MAX results or public APIs are intended.

The change corrects internal memory accounting for grouped byte MIN/MAX accumulators, allowing retained allocations to be reported more accurately.

LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.40230% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.77%. Comparing base (6a31aa8) to head (e7a07f4).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/functions-aggregate/src/min_max/min_max_bytes.rs	95.40%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26154      +/-   ##
==========================================
+ Coverage   82.76%   82.77%   +0.01%     
==========================================
  Files        1147     1147              
  Lines      450354   450850     +496     
  Branches   450354   450850     +496     
==========================================
+ Hits       372714   373179     +465     
- Misses      54932    54947      +15     
- Partials    22708    22724      +16     

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