fix(validators/oci): fail closed on upstream rate limiting (#1281)

## Summary

When the OCI validator's anonymous manifest fetch returns HTTP 429,
return a retryable error instead of `nil`. This aligns OCI with the
fail-closed behavior of the npm / PyPI / NuGet / MCPB validators, and
ensures the `io.modelcontextprotocol.server.name` label-match always
runs before a publish is accepted.

## Test plan

- [x] `go test ./internal/validators/registries/ -run TestValidateOCI`
passes locally
- [x] `gofmt` + `golangci-lint` clean on the changed file
- [ ] CI green

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/validators/registries/oci.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"log"
 	"net/http"
 	"strings"
 	"time"
@@ -106,10 +105,13 @@ func ValidateOCI(ctx context.Context, pkg model.Package, serverName string) erro
 		if errors.As(err, &transportErr) {
 			switch transportErr.StatusCode {
 			case http.StatusTooManyRequests:
-				// Rate limited - skip validation to avoid blocking publishers
-				// This is intentional: we prioritize UX over strict validation during high traffic
-				log.Printf("Skipping OCI validation for %s due to rate limiting", pkg.Identifier)
-				return nil
+				// Fail closed: a 429 means we could not verify the
+				// `io.modelcontextprotocol.server.name` label on the image, which is
+				// the only ownership proof we apply to OCI packages. Returning nil
+				// here would let a publisher bind their server record to an arbitrary
+				// public image they do not control, which is the bug this branch
+				// used to have. Surface the rate-limit as a retryable error instead.
+				return fmt.Errorf("OCI registry is currently rate-limiting validations for '%s'; please retry shortly", pkg.Identifier)
 			case http.StatusNotFound:
 				return fmt.Errorf("OCI image '%s' does not exist in the registry", pkg.Identifier)
 			case http.StatusUnauthorized, http.StatusForbidden: