### What problem does this PR solve?
As title
main fix:
```go
if _, ok := req["meta_data_filter"]; !ok || req["meta_data_filter"] == nil {
req["meta_data_filter"] = map[string]interface{}{}
}
```
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Refactoring
## Summary
- Added Go API route `PUT /api/v1/chats/:chat_id` to align with Python
`PUT /api/v1/chats/<chat_id>` chat update behavior.
- Added Go API route `PATCH /api/v1/chats/:chat_id` to align with Python
`PATCH /api/v1/chats/<chat_id>` partial chat update behavior.
- Added matching handler and service logic for owner checks, tenant
validation, persisted-field filtering, read-only field filtering,
`dataset_ids` to `kb_ids` conversion, and PATCH shallow merge semantics
for `prompt_config` and `llm_setting`.
