feat: centralize LightRAG query config defaults

[Copilot]

LightRAG query parameters had hardcoded defaults scattered across Field(default=...) declarations, requiring updates in multiple places when changing defaults.

Changes

• Add LightRAGQueryDefaults class as single source of truth for all query parameter defaults
• Update LightRAGQueryConfig to reference constants from LightRAGQueryDefaults

Usage

from src.config import LightRAGQueryDefaults

# Access defaults directly
print(LightRAGQueryDefaults.TOP_K)  # 20
print(LightRAGQueryDefaults.CHUNK_TOP_K)  # 10

# Config still works with env var overrides
config = LightRAGQueryConfig()
print(config.top_k)  # Uses default or TOP_K env var

Environment variable overrides continue to work as before.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>LightRAG query parameters hardcoded - difficult to maintain</issue_title>
<issue_description>## 问题描述
LightRAG 查询参数硬编码，维护困难。

受影响的文件

• src/config.py 行 216-225

硬编码的默认值

• top_k=20
• chunk_top_k=10
• max_entity_tokens=6000 等

问题

虽然都有环境变量配置，但每个值都硬编码了默认值，维护困难，每个配置需要在两个地方同时更新。

解决方案

应该将默认值统一放在一个地方管理。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes LightRAG query parameters hardcoded - difficult to maintain #20

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.