feat: Add correlation threshold filtering and maximum limit for retrieved memories

[hijzy]

Description

Correlation Threshold Filtering:
Add a configurable correlation threshold parameter (float, default: 0.0)
If threshold > 0: filter out memories with correlation scores below the threshold
If threshold = 0: return all memories without filtering (backward compatible)

Maximum Retrieval Limit:
Add a configurable maximum number of memories to return (int, default: unlimited/very high)
When retrieving memories, return at most this number of highest-scoring memories

Related Issue (Required): #1005

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Test Script Or Test Steps (please provide)

data = {
  "query": "我国庆想出去玩，帮我推荐个没去过的城市，以及没住过的酒店品牌",
  "user_id": "memos_user_123",
  "conversation_id": "0928",
  "relativity": 0.7
}
headers = {
  "Content-Type": "application/json",
  "Authorization": f"Token {os.environ['MEMOS_API_KEY']}"
}
url = f"{os.environ['MEMOS_BASE_URL']}/search/memory"

res = requests.post(url=url, headers=headers, data=json.dumps(data))

print(f"result: {res.json()}")

Checklist

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have created related documentation issue/PR in MemOS-Docs (if applicable) | 我已在 MemOS-Docs 中创建了相关的文档 issue/PR（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人

Reviewer Checklist

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Made sure Checks passed
• Tests have been provided