Fix vCard 3 inline binary round-tripping

Decode ENCODING=b payloads for PHOTO, LOGO, SOUND, and KEY when parsing
so serializing a parsed card does not base64-encode already encoded wire
data a second time. Add regression coverage for both inline binary and
VALUE=uri round-trips.

AI-Assisted: yes
AI-Tool: OpenAI Codex /gpt-5.4 xhigh

src/vcard/vcard3.nim

@@ -963,6 +963,20 @@ proc readTextValueList(
   result = @[p.readTextValue]
   while seps.contains(p.peek): result.add(p.readTextValue(ignorePrefix = seps))
 
+proc readBinaryValue(
+    p: var VCardParser,
+    isInline: bool,
+    propName: string
+  ): string =
+  let value = p.readValue
+  if not isInline:
+    return value
+
+  try:
+    return base64.decode(value)
+  except ValueError:
+    p.error("invalid base64 value for the " & propName & " content type")
+
 proc parseContentLines*(p: var VCardParser): seq[VC3_Property] =
   result = @[]
 
@@ -1035,12 +1049,13 @@ proc parseContentLines*(p: var VCardParser): seq[VC3_Property] =
       result.add(assignCommon(newVC3_Nickname(value = p.readValue)))
 
     of $pnPhoto:
+      let isInline = params.existsWithValue("ENCODING", "B")
       result.add(newVC3_Photo(
         group = group,
-        value = p.readValue,
+        value = p.readBinaryValue(isInline, "PHOTO"),
         valueType = params.getSingleValue("VALUE"),
         binaryType = params.getSingleValue("TYPE"),
-        isInline = params.existsWithValue("ENCODING", "B")))
+        isInline = isInline))
 
     of $pnBday:
       let valueType = params.getSingleValue("VALUE")
@@ -1121,12 +1136,13 @@ proc parseContentLines*(p: var VCardParser): seq[VC3_Property] =
       result.add(assignCommon(newVC3_Role(value = p.readValue)))
 
     of $pnLogo:
+      let isInline = params.existsWithValue("ENCODING", "B")
       result.add(newVC3_Logo(
         group = group,
-        value = p.readValue,
+        value = p.readBinaryValue(isInline, "LOGO"),
         valueType = params.getSingleValue("VALUE"),
         binaryType = params.getSingleValue("TYPE"),
-        isInline = params.existsWithValue("ENCODING", "B")))
+        isInline = isInline))
 
     of $pnAgent:
       let valueParam = params.getSingleValue("VALUE")
@@ -1182,12 +1198,13 @@ proc parseContentLines*(p: var VCardParser): seq[VC3_Property] =
       result.add(assignCommon(newVC3_SortString(value = p.readValue)))
 
     of $pnSound:
+      let isInline = params.existsWithValue("ENCODING", "B")
       result.add(newVC3_Sound(
         group = group,
-        value = p.readValue,
+        value = p.readBinaryValue(isInline, "SOUND"),
         valueType = params.getSingleValue("VALUE"),
         binaryType = params.getSingleValue("TYPE"),
-        isInline = params.existsWithValue("ENCODING", "B")))
+        isInline = isInline))
 
     of $pnUid:
       result.add(newVC3_UID(group = group, value = p.readValue))
@@ -1204,12 +1221,13 @@ proc parseContentLines*(p: var VCardParser): seq[VC3_Property] =
       result.add(newVC3_Class(group = group, value = p.readValue))
 
     of $pnKey:
+      let isInline = params.existsWithValue("ENCODING", "B")
       result.add(newVC3_Key(
         group = group,
-        value = p.readValue,
+        value = p.readBinaryValue(isInline, "KEY"),
         valueType = params.getSingleValue("VALUE"),
         keyType = params.getSingleValue("TYPE"),
-        isInline = params.existsWithValue("ENCODING", "B")))
+        isInline = isInline))
 
     else:
       if not name.startsWith("X-"):

tests/tvcard3.nim

@@ -153,6 +153,22 @@ suite "vcard/vcard3":
       serialized.contains("SOUND;ENCODING=b;TYPE=WAVE:" & payload)
       serialized.contains("KEY;ENCODING=b;TYPE=PGP:" & payload)
 
+  test "spec: uri-backed binary properties round-trip as uris":
+    let serialized = $parseSingleVCard3(vcard3Doc(
+      "VERSION:3.0",
+      "FN:John Smith",
+      "N:Smith;John;;;",
+      "PHOTO;VALUE=uri:http://example.test/photo.jpg",
+      "LOGO;VALUE=uri:http://example.test/logo.gif",
+      "SOUND;VALUE=uri:http://example.test/sound.wav",
+      "KEY;VALUE=uri:http://example.test/key.asc"))
+
+    check:
+      serialized.contains("PHOTO;VALUE=uri:http://example.test/photo.jpg")
+      serialized.contains("LOGO;VALUE=uri:http://example.test/logo.gif")
+      serialized.contains("SOUND;VALUE=uri:http://example.test/sound.wav")
+      serialized.contains("KEY;VALUE=uri:http://example.test/key.asc")
+
   test "spec: quoted parameter values are accepted":
     let parsed = parseSingleVCard3(vcard3Doc(
       "VERSION:3.0",