fix(helm): support omitting image.registry in grafana-mcp chart (#2125)

## What

Make the grafana-mcp chart render a valid image reference when
`image.registry` is left empty, so the registry can be omitted (e.g. to
pull from a mirror or let the runtime resolve the default registry)
without producing a leading-slash, unpullable reference.

## Why

The image is built inline in `templates/deployment.yaml`:

```gotemplate
image: "{{ .Values.image.registry }}/{{ .Values.image.repository }}:{{ .Values.image.tag }}"
```

The `/` between registry and repository is written unconditionally. If
`image.registry` is set to `""`, the result has a leading slash and is
not a valid image reference, so the pod enters `ImagePullBackOff`:

```
/grafana:latest     # invalid reference
```

There is currently no way to omit the registry and let the container
runtime resolve the repository against its default registry / a
configured mirror.

Omitting the registry is a configuration users legitimately need:

- **Air-gapped / mirrored environments.** Registry mirrors are
configured at the container runtime, which rewrites the host — so chart
values should carry only `repository:tag`.
- **Registry rewriting.** Admission controllers (e.g. Kyverno) prepend
an internal registry, so users leave `registry: ""`.
- **Consistency.** Other charts let you omit the registry; grafana-mcp
silently breaks with `ImagePullBackOff`, which reads like a bug.

> Note: the default `registry: mcp` is intentional and unchanged —
`mcp/grafana` resolves to `docker.io/mcp/grafana` (Docker Hub's `mcp`
namespace). This PR only makes an *empty* registry valid; the default
path still renders `mcp/grafana:latest`.
> This is the same 2-segment `registry/repository:tag` leading-slash
shape as the querydoc chart (#2124), and distinct from the
bundled-PostgreSQL `registry//name:tag` double-slash bug (#2120), which
uses a 3-segment shape. All fixed independently.

## Fix

Introduce a `grafana-mcp.image` helper (the chart currently has none)
that builds the path from non-empty segments via `compact` + `join`, and
call it from the deployment:

```gotemplate
{{- define "grafana-mcp.image" -}}
{{- $img := .Values.image -}}
{{- $parts := compact (list $img.registry $img.repository) -}}
{{- printf "%s:%s" (join "/" $parts) $img.tag -}}
{{- end -}}
```

```gotemplate
# templates/deployment.yaml
image: "{{ include "grafana-mcp.image" . }}"
```

Rendered results:

| registry | repository | tag | result |
|---|---|---|---|
| `mcp` | `grafana` | `latest` | `mcp/grafana:latest` (unchanged) |
| `""` | `grafana` | `latest` | `grafana:latest` (fixed) |

## Tests

Added two cases to `helm/tools/grafana-mcp/tests/deployment_test.yaml`
(developed test-first): a default-image regression guard (the suite had
no image assertion), and the empty-registry path.

```yaml
  - it: should render the default container image
    template: deployment.yaml
    asserts:
      - equal:
          path: spec.template.spec.containers[0].image
          value: mcp/grafana:latest

  - it: should omit empty registry segment in image
    template: deployment.yaml
    set:
      image:
        registry: ""
    asserts:
      - equal:
          path: spec.template.spec.containers[0].image
          value: grafana:latest
      - notMatchRegex:
          path: spec.template.spec.containers[0].image
          pattern: "^/"
```

The empty-registry test fails without this change (`/grafana:latest`)
and passes with it — revert the `deployment.yaml`/`_helpers.tpl` hunks
to reproduce.

## How to verify

```bash
make helm-tools                                       # generate grafana-mcp Chart.yaml
helm unittest helm/tools/grafana-mcp                  # full suite green (incl. new cases)
helm lint helm/tools/grafana-mcp

# test-independent reproduction:
helm template t helm/tools/grafana-mcp --set image.registry="" | grep image:
# before: /grafana:latest
# after:  grafana:latest
```

## Scope

Limited to the grafana-mcp chart's image construction. Empty
`repository` is not made valid here — omitting it leaves no image to
pull. This PR only makes `image.registry` optional.

## Checklist

- [x] Commit is DCO signed-off (`git commit -s`).
- [x] `helm unittest helm/tools/grafana-mcp` green; `helm lint
helm/tools/grafana-mcp` clean.
- [x] Default image still renders `mcp/grafana:latest` (regression
guard).
- [x] Diff limited to `templates/_helpers.tpl`,
`templates/deployment.yaml`, and `tests/deployment_test.yaml` (generated
chart files excluded).
- [x] Test fails without the fix, passes with it.

Signed-off-by: Mike Spinks <mikespinks@gmail.com>

Author: michaelspinks

helm/tools/grafana-mcp/templates/_helpers.tpl

@@ -67,3 +67,12 @@ Create the grafana server URL
 {{- define "grafana-mcp.serverUrl" -}}
 {{- printf "http://%s.%s:%d/mcp" (include "grafana-mcp.fullname" .) .Release.Namespace (.Values.service.port | int) }}
 {{- end }}
+
+{{/*
+Join registry/repository for grafana-mcp image, skipping empty segments, then append tag
+*/}}
+{{- define "grafana-mcp.image" -}}
+{{- $img := .Values.image -}}
+{{- $parts := compact (list $img.registry $img.repository) -}}
+{{- printf "%s:%s" (join "/" $parts) $img.tag -}}
+{{- end -}}

helm/tools/grafana-mcp/templates/deployment.yaml

@@ -35,7 +35,7 @@ spec:
         - name: grafana-mcp
           securityContext:
             {{- toYaml .Values.securityContext | nindent 12 }}
-          image: "{{ .Values.image.registry }}/{{ .Values.image.repository }}:{{ .Values.image.tag }}" 
+          image: {{ include "grafana-mcp.image" . }}
           imagePullPolicy: {{ .Values.image.pullPolicy | default "IfNotPresent" }}
           args:
             - --transport

helm/tools/grafana-mcp/tests/deployment_test.yaml

@@ -100,3 +100,23 @@ tests:
       - equal:
           path: spec.template.spec.containers[0].securityContext.runAsUser
           value: 1000
+
+  - it: should render the default container image
+    template: deployment.yaml
+    asserts:
+      - equal:
+          path: spec.template.spec.containers[0].image
+          value: mcp/grafana:latest
+
+  - it: should omit empty registry segment in image
+    template: deployment.yaml
+    set:
+      image:
+        registry: ""
+    asserts:
+      - equal:
+          path: spec.template.spec.containers[0].image
+          value: grafana:latest
+      - notMatchRegex:
+          path: spec.template.spec.containers[0].image
+          pattern: "^/"          # no leading slash