[iuliiasobolevska]

Summary

PR #968 moved dependency-jar type-mapping merges into CodeGen construction. Embedders that replace config.typeMapping between construction and generate() can therefore lose jar mappings and generate a type that should be mapped.

This change reads dependency mappings during construction but merges them into config.typeMapping at generate() time, matching 8.7.0 behavior. Caller mappings still override jar mappings, earlier jars retain precedence over later jars, and repeated generate() calls remain idempotent.

Validation

• Four focused core type-mapping tests passed on JDK 17 with the in-process Kotlin compiler. They cover post-construction config changes, repeated generation, earlier-jar precedence, dependency mapping, and caller override behavior.
• :graphql-dgs-codegen-core:formatKotlin and git diff --check passed.