Add NGINX ingress rate limiting to protect against traffic spikes (#843)

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## Motivation and Context
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The following is an alternative solution to #833

Goal is to deploy and promote it to prod, then monitor the alerts for a
week to see its effect and reassess. In case it's actual traffic that
we've started blocking we can also up the replicas. I thought it doesn't
make sense to up them now since it would be nice to isolate the result
of this change first.

## How Has This Been Tested?
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tested? -->

## Breaking Changes
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## Types of changes
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the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
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that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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---------

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

deploy/pkg/k8s/registry.go

@@ -246,6 +246,11 @@ func DeployMCPRegistry(ctx *pulumi.Context, cluster *providers.ProviderInfo, env
 			Annotations: pulumi.StringMap{
 				"cert-manager.io/cluster-issuer": pulumi.String("letsencrypt-prod"),
 				"kubernetes.io/ingress.class":    pulumi.String("nginx"),
+			// Rate limiting to protect against abuse
+			// Allows 180 requests/minute (3 req/sec avg), with bursts up to 540 requests
+			"nginx.ingress.kubernetes.io/limit-rpm":              pulumi.String("180"),
+			"nginx.ingress.kubernetes.io/limit-burst-multiplier": pulumi.String("3"),
+			"nginx.ingress.kubernetes.io/limit-req-status-code":  pulumi.String("429"),
 			},
 		},
 		Spec: &networkingv1.IngressSpecArgs{