top_k parameter ignored, always returned page_size results (#12753)

### What problem does this PR solve?
**Backend**
\rag\nlp\search.py
*Before the fix*
The top_k parameter was not applied to limit the total number of chunks,
and the rerank model also uses the exact whole valid_idx rather than
assigning valid_idx = valid_idx[:top] firstly.
*After the fix*
The top_k limit is applied to the total results before pagination, using
a default value of top = 1024 if top_k is not modified.

session.py
*Before the fix:*
When the frontend calls the retrieval API with `search_id`, the backend
only reads `meta_data_filter` from the saved `search_config`. The
`rerank_id`, `top_k`, `similarity_threshold`, and
`vector_similarity_weight` parameters are only taken from the direct
request body. Since the frontend doesn't pass these parameters
explicitly (it only passes `search_id`), they always fall back to
default values:
- `similarity_threshold` = 0.0
- `vector_similarity_weight` = 0.3
- `top_k` = 1024
- `rerank_id` = "" (no rerank)
This means user settings saved in the Search Settings page have no
effect on actual search results.

*After the fix:*
When a `search_id` is provided, the backend now reads all relevant
configuration from the saved `search_config`, including `rerank_id`,
`top_k`, `similarity_threshold`, and `vector_similarity_weight`. Request
parameters can still override these values if explicitly provided,
allowing flexibility. The rerank model is now properly instantiated
using the configured `rerank_id`, making the rerank feature actually
work.



**Frontend** 
\web\src\pages\next-search\search-setting.tsx
*Before the fix*
search-setting.tsx file, the top_k input box is only displayed when
rerank is enabled (wrapped in the rerankModelDisabled condition). If the
rerank switch is turned off, the top_k input field will be hidden, but
the form value will remain unchanged. In other words: - When rerank is
enabled, users can modify top_k (default 1024). - When rerank is
disabled, top_k retains the previous value, but it's not visible on the
interface. Therefore, the backend will always receive the top_k
parameter; it's just that the frontend UI binds this configuration item
to the rerank switch. When rerank is turned off, top_k will not
automatically reset to 1024, but will retain its original value.
*After the fix*
On the contrary, if we switch off the button rerank model, the value
top-k will be reset to 1024. By the way, If we use top-k in an
individual method, rather than put it into the method retrieval, we can
control it separately



Now all methods valid
Using rerank

<img width="2378" height="1565" alt="Screenshot 2026-01-21 190206"
src="https://github.com/user-attachments/assets/fa2b0df0-1334-4ca3-b169-da6c5fd59935"
/>

Not using rerank
<img width="2596" height="1559" alt="Screenshot 2026-01-21 190229"
src="https://github.com/user-attachments/assets/c5a80522-a0e1-40e7-b349-42fe86df3138"
/>




Before fixing they are the same

### Type of change
- Bug Fix (non-breaking change which fixes an issue)

web/src/pages/next-search/search-setting.tsx

@@ -39,7 +39,7 @@ import { IKnowledge } from '@/interfaces/database/knowledge';
 import { cn } from '@/lib/utils';
 import { zodResolver } from '@hookform/resolvers/zod';
 import { X } from 'lucide-react';
-import { useCallback, useEffect, useMemo, useState } from 'react';
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
 import { useForm, useWatch } from 'react-hook-form';
 import { useTranslation } from 'react-i18next';
 import { z } from 'zod';
@@ -226,11 +226,21 @@ const SearchSetting: React.FC<SearchSettingProps> = ({
     control: formMethods.control,
     name: 'search_config.use_rerank',
   });
+
   const aiSummaryDisabled = useWatch({
     control: formMethods.control,
     name: 'search_config.summary',
   });
 
+  // Reset top_k to 1024 only when user actively disables rerank (from true to false)
+  const prevRerankEnabled = useRef<boolean | undefined>(undefined);
+  useEffect(() => {
+    if (prevRerankEnabled.current === true && rerankModelDisabled === false) {
+      formMethods.setValue('search_config.top_k', 1024);
+    }
+    prevRerankEnabled.current = rerankModelDisabled;
+  }, [rerankModelDisabled, formMethods]);
+
   const { updateSearch } = useUpdateSearch();
   const [formSubmitLoading, setFormSubmitLoading] = useState(false);
   const { data: systemSetting } = useFetchTenantInfo();