refactor(ingress)!: skip the Ingress when the HTTP Service is disabled

An Ingress that points at a Service which the chart does not render is broken by definition: the backend reference
cannot resolve and the ingress controller reports the rule as unavailable. The render condition therefore now also
requires `service.http.enabled` and lives in the new `gitea.ingress.enabled` helper, so the same rule can be reused
by other templates instead of being duplicated.

The namespace is taken from `.Release.Namespace` again. The `namespace` value is not a documented chart parameter,
and letting a single resource opt out of the release namespace breaks `helm uninstall` and Argo CD pruning, because
neither tracks objects outside the release namespace.

The `ingress.className` default changes from an empty string to `nginx`. An empty class makes the cluster fall back
to the default IngressClass, which silently produces a different result per cluster; naming the controller the chart
is tested against makes the rendered output predictable.

The scattered ingress suites are consolidated into a single `unittests/helm/ingress/ingress.yaml` that pins the
release name, namespace and appVersion, as required by the testing conventions, and covers the enable/disable matrix,
annotations, labels, TLS and a custom HTTP port.

BREAKING CHANGE: The Ingress is no longer rendered when `service.http.enabled` is `false`. `ingress.className` now
defaults to `nginx` instead of the cluster's default IngressClass. The undocumented `namespace` value no longer
applies to the Ingress.

Co-authored-by: Copilot <copilot@github.com>

.github/copilot-instructions.md

@@ -50,6 +50,7 @@ image:
 - Each test must set all required values explicitly — do not rely on cross-test state.
 - The `values.yaml` file must pass `yamllint`. The configuration is in `.yamllint`. Use `make yamllint` to run the linter.
 - The title of the unit test should clearly describe the scenario being tested. As title must be use a short sentence starting with a capital letter and ending without a period.
+- Each unit test must explicitly set a custom namespace and release name, rather than relying on defaults.
 
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