fix: look up OpenAPI response definition by string status code

router:get-content-type-for-code indexed the string-keyed responses map
(parsed from the OpenAPI JSON, so keys are "200", "404", "default") with
the integer status $code. That only ever matched because of eXist's
pre-conformance map-key coercion, which cast the integer lookup key to
the map's string keyType. eXist-db/exist#6491 makes map keys compare by
op:same-key per XQuery 3.1 section 17.1 (no cross-family coercion), so an
integer key no longer matches a string key: the lookup misses, falls back
to the "application/xml" content type, and a JSON response is serialized
as XML (SENR0001) on the forward-dispatch path.

Look up by string($code) instead. This is correct independent of #6491 -
the OpenAPI keys are strings - and works on both current and pre-#6491
eXist (string-to-string always matched).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 2 people

content/router.xql

@@ -329,7 +329,10 @@ declare %private function router:execute-handler ($base-request as map(*), $use,
 (: content types :)
 
 declare function router:get-content-type-for-code ($config as map(*), $code as xs:integer, $fallback as xs:string) as xs:string {
-    let $response-definition := head(($config?responses?($code), $config?responses?default))
+    (: OpenAPI response keys are strings ("200", "default"); look up the integer status code by its
+     : string form. (Indexing the string-keyed responses map with the integer $code only ever matched
+     : because of eXist's pre-conformance map-key coercion, removed in eXist-db/exist#6491.) :)
+    let $response-definition := head(($config?responses?(string($code)), $config?responses?default))
     let $content := 
         if (exists($response-definition) and $response-definition instance of map(*))
         then $response-definition?content