Fix MXP parser to display invalid tags instead of eating them (#8467)

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#### Brief overview of PR changes/additions
When the MXP parser encounters text inside angle brackets that cannot be
parsed as valid MXP (and is not a custom element), it now displays the
content as-is instead of silently removing it.
#### Motivation for adding to Mudlet
Better handle scenarios where games that do not negotiate MXP still send
it, or when players force-enable MXP on Mudlet's side when the game
isn't sending MXP to begin with.
#### Other info (issues closed, discussion etc)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Zooka <136661366+ZookaOnGit@users.noreply.github.com>

Author: 3 people

src/TMxpProcessor.cpp

@@ -172,6 +172,13 @@ TMxpProcessingResult TMxpProcessor::processMxpInput(char& ch, bool resolveCustom
         }
 
         TMxpTagHandlerResult const result = mMxpTagProcessor.handleTag(mMxpTagProcessor, *mpMxpClient, tag.get());
+
+        // If tag was not handled (not valid MXP and not a custom element), display it as-is
+        if (result == MXP_TAG_NOT_HANDLED) {
+            lastEntityValue = tag->toString();
+            return HANDLER_INSERT_ENTITY_LIT;
+        }
+
         return result == MXP_TAG_COMMIT_LINE ? HANDLER_COMMIT_LINE : HANDLER_NEXT_CHAR;
     }