fix: HashTable dynamic rehash and group count limit for GROUP BY aggregation

[MrPresent-Han]

Summary

• HashTable now dynamically rehashes (doubles capacity) when load factor exceeds 7/8, fixing crash when GROUP BY cardinality > ~1792
• Added configurable queryNode.segcore.maxGroupByGroups (default 100K) to cap total groups and prevent OOM on both C++ (per-segment HashTable) and Go (cross-segment agg reducer) layers
• Added 4 C++ unit tests covering rehash basic/correctness, max groups limit, and multiple rehash rounds

issue: #47569

Test plan

• C++ unit tests: --gtest_filter="*HashTableRehash*:*MaxGroups*"
• E2E: GROUP BY aggregation with >2K unique values should succeed
• E2E: Set queryNode.segcore.maxGroupByGroups to small value, verify clear error message

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[codecov]

Codecov Report

❌ Patch coverage is 92.20779% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.27%. Comparing base (77829de) to head (a09924e).
⚠️ Report is 51 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/core/src/segcore/segcore_init_c.cpp	0.00%	4 Missing ⚠️
internal/agg/aggregate_reducer.go	75.00%	1 Missing and 2 partials ⚠️
pkg/util/paramtable/component_param.go	76.92%	2 Missing and 1 partial ⚠️
internal/core/src/exec/HashTable.cpp	96.42%	1 Missing ⚠️
internal/core/src/segcore/SegcoreConfig.h	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #48174      +/-   ##
==========================================
- Coverage   77.29%   77.27%   -0.03%     
==========================================
  Files        2080     2080              
  Lines      342401   342544     +143     
==========================================
+ Hits       264654   264690      +36     
- Misses      69492    69575      +83     
- Partials     8255     8279      +24     

Components	Coverage Δ
Client	78.62% <ø> (ø)
Core	83.82% <95.23%> (+0.02%)	⬆️
Go	75.47% <78.57%> (-0.05%)	⬇️

Files with missing lines	Coverage Δ
internal/core/src/exec/HashTable.h	98.14% <100.00%> (+7.58%)	⬆️
...l/core/src/exec/operator/query-agg/GroupingSet.cpp	100.00% <100.00%> (ø)
internal/core/unittest/test_query_group_by.cpp	99.88% <100.00%> (+0.01%)	⬆️
internal/util/initcore/query_node.go	79.23% <100.00%> (+0.49%)	⬆️
internal/core/src/exec/HashTable.cpp	97.35% <96.42%> (+3.85%)	⬆️
internal/core/src/segcore/SegcoreConfig.h	80.95% <50.00%> (-1.55%)	⬇️
internal/agg/aggregate_reducer.go	49.16% <75.00%> (+1.70%)	⬆️
pkg/util/paramtable/component_param.go	97.09% <76.92%> (-0.05%)	⬇️
internal/core/src/segcore/segcore_init_c.cpp	17.59% <0.00%> (-0.68%)	⬇️

... and 26 files with indirect coverage changes

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