Drop GET /v0/servers/{serverName} endpoint (#618)

Remove redundant endpoint as suggested in #617. Clients should now use
`GET /v0/servers/{serverName}/versions/latest` instead (using `latest`
as a special version value in the existing endpoint).

This simplifies the API surface and reduces implementation burden for
subregistries, while maintaining a clear migration path for consumers.

## Changes
- Removed `GET /v0/servers/{serverName}` endpoint
- Updated `GET /v0/servers/{serverName}/versions/{version}` to accept
`latest` as special version
- Updated all tests to use new endpoint pattern
- Updated documentation and OpenAPI spec
- Added breaking change notice to CHANGELOG

Fixes #617

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

tests/integration/main.go

@@ -245,7 +245,7 @@ func verifyPublishedServer(serverName string, expected *apiv0.ServerJSON) error
 
 	// URL encode the server name since it contains special characters like "/"
 	encodedName := url.PathEscape(serverName)
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, registryURL+"/v0/servers/"+encodedName, nil)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, registryURL+"/v0/servers/"+encodedName+"/versions/latest", nil)
 	if err != nil {
 		return err
 	}