Spapadop/plt 849 statefulset servicename does not match any generated service

[spapa013]

Summary
This PR is stacked on #26. It should be rebased after #26 is merged

This PR fixes a manifest mismatch where StatefulSet.spec.serviceName referenced devenv-{{.Name}} but no Service with that name was generated.

Changes:

• Added devenv-{{.Name}} headless Service (clusterIP: None) in service.tmpl as the StatefulSet governing service.
• Kept existing devenv-ssh-{{.Name}} NodePort Service unchanged.
• Kept optional devenv-http-{{.Name}} Service and ingress wiring unchanged.
• Updated service golden output and regenerated sample manifests.

Why
StatefulSets are expected to reference a governing Service (typically headless) for stable pod network identity. This removes the structural mismatch and ensures valid StatefulSet/Service topology.

Validation

• go test ./internal/templates -update-golden
• go run ./cmd/devenv generate stelios --config-dir ./developers --output ./build
• go test -v -race -covermode=atomic -coverprofile=./coverage.out ./...
• Validated on the atlab-devenv cluster (stelios) that StatefulSet/service topology is correct (matching serviceName, headless governing service, expected EndpointSlice, unchanged SSH NodePort, and correct HTTP ingress backend), and all checks passed.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/config/validation.go	71.42%	5 Missing and 3 partials ⚠️

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