feat: find component pods by the selectors in status.components

[recharte]

Finds an instance's pods through the per-component selectors the provider runtime now reports in status.components (openeverest/openeverest#3335). status.components[].podRefs no longer exist, so the podRefs path was dead code against current core.

• Listing: each component's pods are listed with its selector. Every selector is narrowed with core.openeverest.io/instance=<instance>, so a broad or wrong selector in the status can never reach another instance's pods.
• Logs and describe: a pod is part of the instance only if one of those selectors matches it. The behaviour is unchanged: any other pod gets a 404.
• Type column: shows the runtime's component name (engine, proxy, configServer) when the pod has it, falling back to app.kubernetes.io/component as before.
• Fallback: instances whose provider doesn't label its pods yet keep using app.kubernetes.io/instance.

Providers: PSMDB already labels its pods (openeverest/provider-percona-server-mongodb#117). PXC and PG follow in openeverest/provider-percona-xtradb-cluster#38 and openeverest/provider-percona-postgresql#45.

Testing

• go vet and go test -race ./... in backend.
• New tests cover listing by selector (pods that only carry the operator label are left out), narrowing a broad selector to the instance, and logs for pods inside and outside the components.
• The tests I touched use testify, which this adds to backend/go.mod; that also moves go.yaml.in/yaml/v3 to v3.0.5.
• The frontend is unchanged.