fix: detect bare pipe alternation as regex in grep_knowledge_files (#26795)

is_regex_pattern only recognized the BRE-escaped form \| and not a bare |,
so a pattern like "Jornak|Silverlake|Orissa" was treated as one literal
string (including the pipe characters) and silently returned no matches.
This contradicted the tool docstring, which explicitly advertises
"error|warn" as an auto-detected regex example, and misled models into
concluding the searched terms were absent from the file.

Checking for a bare | also covers the escaped form, since \| contains |,
and normalize_regex already converts escaped pipes before compilation.
Literal patterns without regex metacharacters are unaffected.

Fixes #26781

backend/open_webui/tools/knowledge_fs.py

@@ -33,9 +33,9 @@ MAX_GREP_MATCHES = 50
 
 
 def is_regex_pattern(pattern: str) -> bool:
-    """Detect if a pattern looks like regex (\|, .*, .+, \d, \w, \s, [...])."""
+    """Detect if a pattern looks like regex (|, .*, .+, \d, \w, \s, [...])."""
     return (
-        '\|' in pattern
+        '|' in pattern
         or '.*' in pattern
         or '.+' in pattern
         or '.?' in pattern