[bugfix] Preserve a literal "+" in resource names on decode

db:to-display decoded names with xmldb:decode-uri, which form-decodes "+"
to a space (the x-www-form-urlencoded convention; eXist-db/exist#1824).
But a "+" in a stored name is always a literal "+" -- spaces are stored as
%20 -- and db:to-stored (fn:iri-to-uri) leaves "+" untouched on the encode
side, so a name like "naïve+test.xml" stored correctly but read back as
"naïve test.xml".

Protect a literal "+" as %2B before xmldb:decode-uri so it decodes back to
"+", restoring symmetry with the encode side. This mirrors what
URIUtils.decodeForURI (the core fix in eXist-db/exist#6451) does, applied
at the API layer so it is correct independent of the core build, and
forward-compatible once #6451 lands. Spaces (%20) are unaffected.

Verified end-to-end against a live instance: "naïve+test.xml" stores as
na%C3%AFve+test.xml on disk, lists and reads back as naïve+test.xml. Adds
the "+" case to the Cypress awkward-name coverage.

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

Author: joewiz

modules/db.xqm

@@ -57,7 +57,16 @@ declare %private function db:to-display($path as xs:string?) as xs:string? {
     if (empty($path)) then $path
     else string-join(
         for $segment in tokenize($path, "/")
-        return if ($segment eq "") then "" else xmldb:decode-uri(xs:anyURI($segment)),
+        return
+            if ($segment eq "") then ""
+            (: xmldb:decode-uri form-decodes "+" to a space (the x-www-form-urlencoded
+             : convention; eXist-db/exist#1824), but a "+" in a STORED name is always a
+             : literal "+" -- spaces are stored as %20. Protect literal "+" as %2B so it
+             : decodes back to "+", staying symmetric with db:to-stored / fn:iri-to-uri,
+             : which leaves "+" untouched on the encode side. This mirrors what
+             : URIUtils.decodeForURI (the core fix in eXist-db/exist#6451) does, applied
+             : at the API layer so it is correct independent of the core build. :)
+            else xmldb:decode-uri(xs:anyURI(replace($segment, "\+", "%2B"))),
         "/"
     )
 };

src/test/cypress/e2e/db.cy.js

@@ -817,7 +817,9 @@ describe('/api/db', () => {
     // "café déjà.xml", never "caf%C3%A9...". eXist stores names percent-encoded,
     // so db.xqm encodes on input and decodes on output. The other db tests use
     // ASCII names and so don't exercise this boundary.
-    const NAMES = ['café déjà.xml', "o'brien.xml"]; // non-ASCII + space; sub-delim apostrophe (xmldb:store leaves it literal)
+    // non-ASCII + space; sub-delim apostrophe (xmldb:store leaves it literal); literal "+"
+    // (must survive: stored as a literal "+", decoded back to "+", NOT form-decoded to a space)
+    const NAMES = ['café déjà.xml', "o'brien.xml", 'naïve+test.xml'];
 
     before(() => {
       cy.request({