test(embedder): isolate Ark factory tests from optional SDK

[RerankerGuo]

Description

The Ark factory unit tests mock ArkEmbedder.embed, but constructing the embedder still executes the optional volcenginesdkarkruntime dependency guard. As a result, these factory tests fail in a normal core/test environment even though they never exercise the Ark SDK.

This change mocks the constructor boundary as well as embed, keeping the tests focused on factory selection and single/batch forwarding. Production dependency validation remains covered by the real constructor.

Related Issue (Required): N/A — test isolation regression found on current main.

Type of change

• Bug fix (test-suite reliability)

How Has This Been Tested?

• Before: pytest tests/configs tests/embedders tests/multi_mem_cube -q failed both Ark tests without the optional SDK.
• After: .venv/bin/pytest tests/embedders/test_ark.py -q (2 passed).
• .venv/bin/ruff check tests/embedders/test_ark.py
• .venv/bin/ruff format --check tests/embedders/test_ark.py

Checklist

• Self-reviewed
• Tests prove the regression is fixed
• No production code or dependency metadata changed

[Memtensor-AI]

🤖 Open Code Review

Target: PR #2215
Task: 6ae0aeed3ed9721c
Base: main
Head: test/ark-factory-optional-sdk

✅ OpenCodeReview: No comments generated. Looks good to me.

Generated by cloud-assistant via Open Code Review.

[Memtensor-AI]

✅ Automated Test Results: PASSED

All tests passed (2/2 executed). memos_python_core/changed-repo-python: 2/2. Duration: 5s [advisory, non-gating] AI-generated tests on branch test/auto-gen-6ae0aeed3ed9721c-20260805142200: 37/38 passed, 1 failed — these do NOT affect the PR verdict; review the branch manually.

Branch: test/ark-factory-optional-sdk