feat: implement DNS namespace validation and remote URL uniqueness checks (#306)

## Summary

- Add reverse-DNS namespace format validation for server names
(namespace/name)
- Validate remote URLs match the DNS namespace derived from hostname  
- Prevent duplicate remote URLs across different servers
- Support localhost/development URLs without namespace restrictions
- Add comprehensive test coverage for validation logic
- Update database layer to support remote_url filtering

Fixes #180 by ensuring proper namespace governance and preventing URL
conflicts.

---------

Co-authored-by: Claude <noreply@anthropic.com>

internal/database/database.go

@@ -13,7 +13,7 @@ var (
 	ErrAlreadyExists     = errors.New("record already exists")
 	ErrInvalidInput      = errors.New("invalid input")
 	ErrDatabase          = errors.New("database error")
-	ErrInvalidVersion    = errors.New("invalid version: cannot publish older version after newer version")
+	ErrInvalidVersion    = errors.New("invalid version: cannot publish duplicate version")
 	ErrMaxServersReached = errors.New("maximum number of versions for this server reached (10000): please reach out at https://github.com/modelcontextprotocol/registry to explain your use case")
 )
 

internal/database/memory.go

@@ -42,7 +42,7 @@ func NewMemoryDB(e map[string]*model.ServerDetail) *MemoryDB {
 }
 
 
-// List retrieves ServerRecord entries with optional filtering and pagination
+//nolint:cyclop // Complexity from filtering logic is acceptable for memory implementation
 func (db *MemoryDB) List(
 	ctx context.Context,
 	filter map[string]any,
@@ -89,6 +89,18 @@ func (db *MemoryDB) List(
 				if string(entry.ServerJSON.Status) != value.(string) {
 					include = false
 				}
+			case "remote_url":
+				found := false
+				remoteURL := value.(string)
+				for _, remote := range entry.ServerJSON.Remotes {
+					if remote.URL == remoteURL {
+						found = true
+						break
+					}
+				}
+				if !found {
+					include = false
+				}
 			}
 		}
 

internal/database/postgres.go

@@ -76,6 +76,10 @@ func (db *PostgreSQL) List(
 			whereClause += fmt.Sprintf(" AND s.status = $%d", argIndex)
 			args = append(args, v)
 			argIndex++
+		case "remote_url":
+			whereClause += fmt.Sprintf(" AND EXISTS (SELECT 1 FROM jsonb_array_elements(CASE WHEN jsonb_typeof(s.remotes) = 'array' THEN s.remotes ELSE '[]'::jsonb END) AS remote WHERE remote->>'url' = $%d)", argIndex)
+			args = append(args, v)
+			argIndex++
 		}
 	}