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>

docs/modelcontextprotocol-io/package-types.mdx

@@ -131,6 +131,7 @@ For Docker/OCI images, the MCP Registry currently supports:
 
 - Docker Hub (`docker.io`)
 - GitHub Container Registry (`ghcr.io`)
+- Quay.io (`quay.io`)
 - Google Artifact Registry (any `*.pkg.dev` domain)
 - Azure Container Registry (`*.azurecr.io`)
 - Microsoft Container Registry (`mcr.microsoft.com`)
@@ -156,7 +157,7 @@ Docker/OCI images use `"registryType": "oci"` in `server.json`. For example:
 }
 ```
 
-The format of `identifier` is `registry/namespace/repository:tag`. For example, `docker.io/user/app:1.0.0` or `ghcr.io/user/app:1.0.0`. The tag can also be specified as a digest.
+The format of `identifier` is `registry/namespace/repository:tag`. For example, `docker.io/user/app:1.0.0`, `ghcr.io/user/app:1.0.0`, or `quay.io/myorg/my-mcp-server:1.0.0`. The tag can also be specified as a digest.
 
 ### Ownership Verification