Fix missing type casting in AGG group qualifier encoding

[zipdoki]

Summary

Cast AGG predicate values using schema field types resolved at entity construction.

Closes #227

What changed

• Group.Field gains a type: PrimitiveType? field. bucketOrGet() uses it to cast when no bucket is present.
• LabelEntity factory methods resolve DataType → PrimitiveType from the schema. Resolution always runs to avoid stale values.
• DECIMAL fields throw an explicit error (not yet supported in PrimitiveType).
• Added unit tests for bucketOrGet() branching and an E2E test covering bucket-only, String+bucket, and Long+bucket group configurations.

How to test

# Unit tests — bucketOrGet() branching logic
./gradlew :core:test --tests "com.kakao.actionbase.core.metadata.common.GroupFieldTest"

# E2E tests — full AGG query flow (mutation → query) for 3 group types
./gradlew :server:test --tests "com.kakao.actionbase.server.api.graph.v3.EdgeAggQueryE2ETest"

# Full build
./gradlew build

[zipdoki]

@em3s
DECIMAL group fields are not yet supported — PrimitiveType has no DECIMAL variant. Tracked for a follow-up.

[em3s]

Left a comment on #227.