fix: override default fleet-agent metrics and health bind addresses

[anmazzotti]

What this PR does / why we need it:

Original finding: #2154 (comment)

I found out that on GKE the fleet-agent fails to initialize correctly since the metrics port 8080 is already bound (also see the reserved port list).

{"level":"error","ts":"2026-02-26T15:32:50Z","logger":"setup","msg":"problem running manager","error":"failed to start metrics server: failed to create listener: listen tcp :8080: bind: address already in use","stacktrace":"github.com/rancher/fleet/internal/cmd/agent.start\n\t/home/runner/_work/fleet/fleet/internal/cmd/agent/operator.go:181\ngithub.com/rancher/fleet/internal/cmd/agent.(*FleetAgent).Run.func1\n\t/home/runner/_work/fleet/fleet/internal/cmd/agent/root.go:143"}
{"level":"error","ts":"2026-02-26T15:32:50Z","logger":"setup","msg":"failed to start agent","error":"failed to start metrics server: failed to create listener: listen tcp :8080: bind: address already in use","stacktrace":"github.com/rancher/fleet/internal/cmd/agent.(*FleetAgent).Run.func1\n\t/home/runner/_work/fleet/fleet/internal/cmd/agent/root.go:144"}
{"level":"error","ts":"2026-02-26T15:32:50Z","logger":"controller-runtime.source.Kind","msg":"failed to get informer from cache","error":"Timeout: failed waiting for *v1alpha1.BundleDeployment Informer to sync","stacktrace":"sigs.k8s.io/controller-runtime/pkg/internal/source.(*Kind[...]).Start.func1.1\n\t/home/runner/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/source/kind.go:80\nk8s.io/apimachinery/pkg/util/wait.loopConditionUntilContext.func1\n\t/home/runner/go/pkg/mod/k8s.io/apimachinery@v0.35.0/pkg/util/wait/loop.go:53\nk8s.io/apimachinery/pkg/util/wait.loopConditionUntilContext\n\t/home/runner/go/pkg/mod/k8s.io/apimachinery@v0.35.0/pkg/util/wait/loop.go:54\nk8s.io/apimachinery/pkg/util/wait.PollUntilContextCancel\n\t/home/runner/go/pkg/mod/k8s.io/apimachinery@v0.35.0/pkg/util/wait/poll.go:33\nsigs.k8s.io/controller-runtime/pkg/internal/source.(*Kind[...]).Start.func1\n\t/home/runner/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/source/kind.go:68"}
{"level":"error","ts":"2026-02-26T15:33:05Z","logger":"clusterstatus","msg":"failed to report initial cluster status","cluster":"cluster-gke-pq8v6b","interval":900,"error":"client rate limiter Wait returned an error: context canceled","stacktrace":"github.com/rancher/fleet/internal/cmd/agent/clusterstatus.Ticker.func1\n\t/home/runner/_work/fleet/fleet/internal/cmd/agent/clusterstatus/ticker.go:42"}

This can be configured using fleet-agent environment variables, however there are two issues:

1. The FleetAddonConfig is embedded in the rancher-turtles-providers chart (and previously it was embedded in the turtles one)
2. CAAPF does not allow changing the configuration per Cluster (see Allow different agent configurations per Cluster cluster-api-addon-provider-fleet#428)

So I see no other way than changing this for all Clusters and for all rancher-turtles-providers chart users.
This is an opinionated choice, however since we also use the hostNetwork setting, trying to bind to 18080 and 18081 is probably safer in most cases.

This however has the consequence of rolling out the fleet-agent on already provisioned Clusters to bind to the newly set ports, which is surely going to be an unexpected change for current users.
Chart configuration values have been added so that users can default back to 8080 and 8081 if they wish to.

Test run that includes this change: https://github.com/rancher/turtles/actions/runs/22565380279/job/65360516383

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests

[yiannistri]

Tested with

 helm template rancher-turtles-providers

and

 helm template rancher-turtles-providers --set extras.addonFleet.config.enabled=false

Thank you!