fix: batch missing search embeddings

[RerankerGuo]

Description

Related Issue (Required): Fixes #1482

Batches missing embedding backfills during API search MMR deduplication.

SearchHandler._extract_embeddings() previously sent every memory without cached embedding in one embedder call. Providers such as DashScope text-embedding-v4 reject batches larger than 10, so a search result with many missing embeddings could fail during deduplication. This change caps each missing-embedding call at 10 documents while preserving the returned embeddings in the original memory order.

No public API, request/response model, OpenAPI, dependency, or database schema changes.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Unit Test
• Test Script Or Test Steps (please provide)
• Pipeline Automated API Test (please provide)

Commands run:

python3 -m py_compile src/memos/api/handlers/search_handler.py tests/api/test_search_handler_embedding_batches.py

Result: passed.

Also attempted:

PYTHONPATH=src python3 -m pytest tests/api/test_search_handler_embedding_batches.py -q
make format

These are blocked in the local environment because pytest and poetry are not installed. Direct test-function execution is also blocked because pydantic is not installed.

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

[Memtensor-AI]

Automated Test Results: QUEUED

This PR was retargeted to dev-v2.0.22; cloud test-engine rerun has been queued.

• Run: tr-7c2dae84-562 on cloud test-engine 10010
• Branch tested: refs/pull/2041/head

I will post the final pass/fail result when the run completes.

[Memtensor-AI]

Automated Test Results: PASSED

Cloud test-engine rerun against dev-v2.0.22 completed successfully.

• Run: tr-7c2dae84-562 on cloud test-engine 10010
• memos_python_core/changed-repo-python: 1 passed, 0 failed, 0 skipped

Manual code review is still required before merge.