chore: update Go templates to 1.25

[xgtcode]

Summary

• Upgrade Go templates to Go 1.25.
• Switch Go builder/runtime images to agentkit-prod-public-cn-beijing.cr.volces.com.
• Upgrade basic_go / a2a_go to veadk-go v1.0.0 and google.golang.org/adk v1.2.0.
• Update template code for the new *apps.ApiConfig API.
• Add tests for Go base image defaults and generated Dockerfile contents.

Test

uv run --with pytest pytest \
  tests/toolkit/docker/test_base_image_defaults.py \
  tests/toolkit/builders/test_local_docker_build.py \
  tests/toolkit/executors/test_init_executor.py \
  -q