fix: optimize reranking module robustness and bug fixes (#14264)

## Description
This PR fixes critical bugs and improves the robustness of the RAG
reranking module while maintaining **100% backward compatibility** with
all existing functionality and providers.

## Key Changes
1. **Network Stability**: Added 30s timeout to all API requests to
prevent service blocking
2. **Boundary Protection**: Added empty query/text validation for all
rerank models
3. **Response Fault Tolerance**: Replaced hardcoded key access with
`.get()` to avoid KeyError crashes
4. **Bug Fixes**:
   - Fixed `Ai302Rerank` (completely non-functional before)
   - Fixed `GPUStackRerank` incorrect exception catching
   - Fixed `_normalize_rank` empty array crash
5. **Code Specification**: Added type annotations, standardized
unimplemented class prompts

## Compatibility
- ✅ No changes to any class/method names
- ✅ All rerank providers (Jina/Cohere/NVIDIA/HuggingFace etc.) work as
before
- ✅ No breaking changes, zero impact on existing workflows

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Refactoring

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/llm/rerank_model.py

@@ -435,6 +435,7 @@ class HuggingfaceRerank(Base):
 
         for i in range(0, len(texts), batch_size):
             try:
+                # Fix: Add request timeout
                 res = requests.post(
                     endpoint, headers={"Content-Type": "application/json"}, 
                     json={"query": query, "texts": texts[i:i+batch_size], "raw_scores": False, "truncate": True},