Support QDRANT_URL for Python Qdrant configuration

[haojingcn]

Fixes #5378

This PR adds conservative QDRANT_URL / QDRANT_API_KEY environment fallback support for the Python Qdrant vector store configuration.

The fallback is only used when no explicit connection method is provided (client, url, host + port, or path), so existing users who configure host + port keep the same behavior.

Docs now recommend using url: "http://..." or QDRANT_URL for self-hosted HTTP Qdrant with API key authentication, because Python qdrant-client treats host + port + api_key as HTTPS by default.

Tests:

• env MEM0_DIR=/private/tmp/mem0-oss-pr-test-home python3 -m pytest tests/configs/test_qdrant_config.py
• python3 -m py_compile mem0/configs/vector_stores/qdrant.py mem0/vector_stores/qdrant.py

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