feat(oci): add support for Quay.io registry (#1150)

Adds support for the Quay.io container registry to the OCI validator.
This allows MCP servers hosted on Quay.io to be registered and validated
by the official registry.

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->

Quay.io is a major, trusted public OCI registry widely used by the
cloud-native and open-source communities. Currently, attempting to
publish a server using a Quay image results in an unsupported OCI
registry error. This change enables developers to use Quay.io as a
first-class hosting option for their MCP server images.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
- **Unit Tests:** Updated `internal/validators/registries/oci_test.go`
to transition Quay.io from a rejected test case to an allowed one.
- **Pattern Matching:** Verified that the allowlist correctly handles
both specific image identifiers and general pattern matching for the
`quay.io` host.
- **Local Validation:** Ran `make check` to ensure no regressions in the
existing OCI validation logic.

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
No. This is an additive change that expands the allowlist of supported
registries. Existing configurations are unaffected.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->
The implementation follows the existing pattern for OCI registries:
1. Added a `RegistryURLQuay` constant to `pkg/model` for consistency.
2. Hardcoded `quay.io` into the `allowedOCIRegistries` map in the
validator to minimize architectural complexity.
3. Updated all relevant documentation files (`package-types.mdx`,
`generic-server-json.md`, and `official-registry-requirements.md`) to
ensure the source of truth is updated across the project.
4. Validation remains scoped to public images; no private registry
authentication was added for this host.

Signed-off-by: Mikel Olasagasti Uranga <mikel@olasagasti.info>
Co-authored-by: Radoslav Dimitrov <radoslav@stacklok.com>

internal/validators/registries/oci.go

@@ -33,6 +33,8 @@ var allowedOCIRegistries = map[string]bool{
 	"index.docker.io":      true, // Docker Hub index
 	// GitHub Container Registry
 	"ghcr.io": true,
+	// Red Hat Quay
+	"quay.io": true,
 	// Microsoft Container Registry
 	"mcr.microsoft.com": true,
 	// Google Artifact Registry (*.pkg.dev pattern handled in isAllowedRegistry)
@@ -49,6 +51,7 @@ var allowedOCIRegistries = map[string]bool{
 // Supported registries:
 //   - Docker Hub (docker.io)
 //   - GitHub Container Registry (ghcr.io)
+//   - Quay.io (quay.io)
 //   - Google Artifact Registry (*.pkg.dev)
 //   - Microsoft Container Registry (mcr.microsoft.com)
 func ValidateOCI(ctx context.Context, pkg model.Package, serverName string) error {

internal/validators/registries/oci_test.go

@@ -66,6 +66,13 @@ func TestValidateOCI_RegistryAllowlist(t *testing.T) {
 			errorMsg:          "missing required annotation",
 			mustNotContainMsg: "unsupported OCI registry",
 		},
+		{
+			name:              "Quay.io should be allowed",
+			identifier:        "quay.io/prometheus/node-exporter:v1.7.0",
+			expectError:       true,
+			errorMsg:          "missing required annotation",
+			mustNotContainMsg: "unsupported OCI registry",
+		},
 		// Removed ACR test with non-existent host - ACR support is tested elsewhere
 
 		// Disallowed registries
@@ -75,12 +82,6 @@ func TestValidateOCI_RegistryAllowlist(t *testing.T) {
 			expectError: true,
 			errorMsg:    "unsupported OCI registry",
 		},
-		{
-			name:        "Quay.io should be rejected",
-			identifier:  "quay.io/test/image:latest",
-			expectError: true,
-			errorMsg:    "unsupported OCI registry",
-		},
 		{
 			name:        "ECR Public should be rejected",
 			identifier:  "public.ecr.aws/test/image:latest",
@@ -134,8 +135,9 @@ func TestValidateOCI_RegistryAllowlist(t *testing.T) {
 }
 
 func TestValidateOCI_RegistryPatterns(t *testing.T) {
-	// This test verifies registry pattern matching (wildcards like *.azurecr.io and *.pkg.dev)
-	// without relying on external images that may not exist
+	// Verifies allowlist behavior: wildcard hosts (*.azurecr.io, *.pkg.dev) and fixed hosts
+	// like quay.io. shouldFail=false means the error must not be "unsupported OCI registry"
+	// (validation may still error later, for example missing annotation or image not found).
 	tests := []struct {
 		name       string
 		identifier string
@@ -156,6 +158,12 @@ func TestValidateOCI_RegistryPatterns(t *testing.T) {
 			identifier: "us-west1-docker.pkg.dev/project/repo/image:tag",
 			shouldFail: false,
 		},
+		{
+			name:       "Quay.io host should be allowed",
+			identifier: "quay.io/nonexistent/mcp-registry-fake-repo:v1",
+			// Past allowlist; fake repo typically yields "does not exist", not unsupported registry
+			shouldFail: false,
+		},
 		{
 			name:       "GCR should be rejected at registry check",
 			identifier: "gcr.io/project/image:latest",