enhance: reduce group-by map allocation churn

[lyang24]

issue: #53218

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✅ CI Loop Results be49e91

Stage	Result	Duration	Tests
✅ Build	SUCCESS	17.2min	-
✅ Code-Check	SUCCESS	7.8min	-
✅ UT-Integration	SUCCESS	28.9min	-
✅ UT-GO	SUCCESS	25.9min	-
✅ UT-CPP-Cov	SUCCESS	66.5min	8819 total, 8819 passed, 0 failed

Total: 91min | Pipeline | Artifacts

Overall Coverage: 75.1%
Diff Coverage: CPP 100.0% (7 hit, 0 miss, 7 measurable lines, 54 unmeasured)
Diff Coverage HTML: view changed lines
Total Patch Coverage: 100.0% (7/7 measurable lines, 54 unmeasured)

[yhmo]

internal/core/src/exec/operator/search-groupby/SearchGroupByOperator.cpp line:211
Low ---- The repository's CODE_REVIEW.md asks that performance PRs list benchmark results; this change reworks the group-by hot path (Push capacity probe + result-vector reserve) but the PR description contains no benchmark data, so the claimed allocation-churn reduction is not yet verifiable. Consider adding a before/after measurement. Non-blocking.