buildJobSpec / buildWorkerSpec image field — accept structured map shape (parity with parent service)

[intel352]

Summary
internal/drivers/app_platform_buildspec.go::buildJobSpec and buildWorkerSpec parse the image field via strFromConfig + ParseImageRef only:

if imgStr := strFromConfig(m, "image", ""); imgStr != "" {
img, err := ParseImageRef(imgStr)
if err == nil {
job.Image = img
}
}

The parent service's image is parsed via imageSpecFromConfig (line ~488), which accepts BOTH string AND structured map shapes:

func imageSpecFromConfig(cfg map[string]any) (*godo.ImageSourceSpec, error) {
switch v := cfg["image"].(type) {
case string:
return ParseImageRef(v)
case map[string]any:
// ... build from registry_type/repository/tag/registry
}
}

When operators use the structured map form on a Job (or Worker), it's silently dropped, and DO API rejects with:

job missing source spec (image, git, github, gitlab or bitbucket)

Surfaced
core-dump#173 added a postgres:18 pre_deploy db-reset job using:

image:
registry_type: DOCKER_HUB
repository: library/postgres
tag: "18"

Deploy run 25270989944 rejected the spec. Workaround in core-dump#174: switch to string form docker.io/library/postgres:18. But the inconsistency between parent-service and job/worker shape parsing should be fixed in the plugin.

Fix
Make buildJobSpec and buildWorkerSpec call imageSpecFromConfig (the parity helper) instead of inline strFromConfig + ParseImageRef.

Test plan
Test that structured-map image shape on a job (and worker) constructs the right *godo.ImageSourceSpec
Test that string image shape still works (regression)
Test that empty image yields nil image (regression)