[test] Avoid Chroma model download in memory test

[joeyutong]

Purpose of change

• replace the test-only mock embedding with a deterministic local embedding
• avoid implicit ChromaDB default embedding downloads in non-Ollama test runs
• keep the unit test focused on long-term memory add/get/search behavior

Tests

• test_get_memory_set
• test_add_and_get
• test_search
• test_compact skipped; depends on an Ollama server

Documentation

• doc-needed
• doc-not-needed
• doc-included

[wenjin272]

Hi, @joeyutong. According to the 0.3 roadmap, we will remove the current implementation of long-term memory and introduce mem0 backend. Consequently, this test will also be removed in the near future.

[joeyutong]

@wenjin272 Thanks for your update. I'll close this PR.