KQL search can't escape quotes, a trailing backslash causes a 500 #3666

Describe the bug

KQL search has no working way to escape " (or a trailing \) inside a quoted value. Clients that build queries from user input (e.g. the web search bar) can't search for terms that contain quotes, and a trailing backslash crashes the request.

Steps to reproduce

1. Upload a file named Angebot "Quote" Test.txt with the content Der Kunde sagte "Quotewort" im Gespraech.
2. Send a WebDAV REPORT search to /remote.php/dav/spaces/ with these patterns:
   • name:"*\"Quote\"*"
   • content:"\"Quotewort\""
   • content:"foo\"

Expected behavior

\" is treated as an escaped quote and the file is found. Invalid queries return a 400.

Actual behavior

• name:"*\"Quote\"*" → 207, no results
• content:"\"Quotewort\"" → 500
• content:"foo\" → 500

Without quotes (name:"*Quote*", content:"Quotewort") the file is found.

Setup

OpenCloud 8.0.1 (rolling), docker-compose.yml from opencloud-eu/web, full text search with Tika (SEARCH_EXTRACTOR_TYPE=tika).

Additional context

Found while working on the web search (opencloud-eu/web#3558).