fix(vector_stores): use platform user-data dir instead of /tmp/{provider}

[alvinttang]

Refs #4279.

The default vector store path was /tmp/{provider}. Three reasons that breaks:

• macOS LaunchAgents: sandbox often blocks /tmp writes; /tmp is periodically cleaned by the OS, so any persisted index gets wiped.
• systemd services with PrivateTmp= or noexec /tmp.
• Windows: no /tmp at all.
• Docker: /tmp is ephemeral unless mounted as a volume, so embedded stores silently lose data on restart.

The maintainer comment on the issue (here) said the right move is to allow an explicit path and pick a more robust default. This PR does both.

New resolution order:

1. MEM0_DATA_DIR environment variable (explicit override).
2. Platform convention via stdlib (no new dep):
   • macOS: ~/Library/Application Support/mem0
   • Windows: %LOCALAPPDATA%/mem0
   • Linux/BSD: $XDG_DATA_HOME/mem0 or ~/.local/share/mem0
3. Provider name appended as the subdir, so faiss keeps its own folder.

Explicit path= in the user config still wins over both the env var and the default, so anyone already setting path sees no change.

RED

def test_default_path_is_not_tmp():
    cfg = VectorStoreConfig(provider="faiss", config={})
    assert not cfg.config.path.startswith("/tmp/")
E   AssertionError: default path still falls back to /tmp: /tmp/faiss

GREEN

test_default_path_is_not_tmp                          PASSED
test_env_var_override                                 PASSED
test_explicit_path_wins                               PASSED
test_macos_default_uses_application_support           PASSED
test_linux_default_respects_xdg_data_home             SKIPPED (linux-only)
4 passed, 1 skipped in 0.52s

tests/configs/ also still passes. The other tests/vector_stores/test_* files require optional backends (chromadb, pgvector, mongodb, etc.) so I couldn't run them locally without installing each, but the only line I touched in configs.py is the path default, which never reaches those backend paths.

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[kaushikmk186-sudo]

Good fix — the /tmp default is a real ops hazard, especially in Docker and systemd environments where silent data loss on restart is exactly the kind of thing that surfaces as a retrieval quality problem rather than a write-path error.

One related note: if you're running an embedded vector store and also hitting the silent embedding drop issue (#5245), the combination is particularly bad — writes silently fail at the embedding layer, and any that do land are potentially wiped on restart. vector-router (https://github.com/Adelagric/vector-router/releases/tag/v0.1.0) sits between the embedding provider and the store and surfaces per-item failures before they reach the store, so it composes with this fix rather than depending on it.

Apache 2.0, drop-in via OPENAI_BASE_URL if useful while both PRs work through review.