fix: preserve uploaded file attachments after subsequent assistant messages (#13993)

## Problem

When a user uploads a file attachment in their first message (Q1) and
then sends a follow-up message (Q2) that triggers a backend response,
the uploaded file attachment disappears from Q1 in the chat UI.

Fixes #13959

## Root Cause

In `single-chat-box.tsx`, a `useEffect` hook syncs `derivedMessages`
from `conversation?.messages` whenever the conversation data changes
(e.g., after a new assistant reply arrives):

```typescript
useEffect(() => {
  const messages = conversation?.messages;
  if (Array.isArray(messages)) {
    setDerivedMessages(messages); // ← overwrites local state
  }
}, [conversation?.messages, setDerivedMessages]);
```

The problem is that `conversation.messages` comes from the server, which
stores messages as plain JSON. Browser `File` objects (uploaded by the
user) cannot be serialized to JSON, so they are never stored on the
server. Each time the server data is applied to local state, the `files`
array on the user's first message is lost.

## Fix

Instead of replacing the local messages wholesale, preserve any `files`
entries from the previous local state by ID before applying the server
data:

```typescript
useEffect(() => {
  const messages = conversation?.messages;
  if (Array.isArray(messages)) {
    setDerivedMessages((prevMessages) => {
      const filesMap = new Map(
        prevMessages
          .filter((m) => m.files?.length)
          .map((m) => [m.id, m.files]),
      );
      if (filesMap.size === 0) {
        return messages;
      }
      return messages.map((m) => ({
        ...m,
        files: filesMap.get(m.id) ?? m.files,
      }));
    });
  }
}, [conversation?.messages, setDerivedMessages]);
```

This is a minimal, targeted fix: when there are no local files to
preserve the behavior is identical to before (early return with plain
assignment). When local file objects exist they are re-attached to the
corresponding server messages by ID.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved search query processing to properly handle special characters
and apostrophes in search terms and synonyms.
* Fixed chat message file attachments to persist when syncing with
server.

* **Refactor**
  * Simplified OCR detection return values by removing timing metadata.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: ximi <octo-patch@github.com>

web/src/pages/next-chats/chat/chat-box/single-chat-box.tsx

@@ -58,7 +58,23 @@ export function SingleChatBox({
   useEffect(() => {
     const messages = conversation?.messages;
     if (Array.isArray(messages)) {
-      setDerivedMessages(messages);
+      setDerivedMessages((prevMessages) => {
+        // Preserve uploaded file objects from local state that the server doesn't
+        // persist (e.g. File instances). Build a map of message id → files from
+        // the current local state so they survive when server data is applied.
+        const filesMap = new Map(
+          prevMessages
+            .filter((m) => m.files?.length)
+            .map((m) => [m.id, m.files]),
+        );
+        if (filesMap.size === 0) {
+          return messages;
+        }
+        return messages.map((m) => ({
+          ...m,
+          files: filesMap.get(m.id) ?? m.files,
+        }));
+      });
     }
   }, [conversation?.messages, setDerivedMessages]);