enhance: support search aggregation search size (#49892)

## Summary
- Add per-level `search_size` handling for search aggregation context
building.
- Preserve `size` as the response bucket limit while using `search_size`
for downstream candidate bucket budget derivation.
- Bump the root module milvus-proto dependency to the version that
contains `SearchAggregationSpec.search_size`.

## Test Plan
- [x] `git diff --check trunk/master...HEAD`
- [x] `go test -tags dynamic,test -gcflags=\"all=-N -l\" -count=1
./internal/proxy/search_agg/...`

related: #48971

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Signed-off-by: MrPresent-Han <chun.han@gmail.com>
Co-authored-by: MrPresent-Han <chun.han@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>