fix: bail out of get_provider('local') when sentence-transformers is missing

[mvanhorn]

Summary

embed_graph_tool returned {ok: true, embeddings: 0} on workspaces without sentence-transformers because get_provider("local") only caught ImportError from the LocalEmbeddingProvider constructor. The check existed (_check_available()) but wasn't consulted before the constructor ran, so the silent fallback path swallowed the unavailability and returned a no-op provider.

Add an early _check_available() short-circuit at the top of get_provider so the call returns None and surfaces "local provider unavailable" to the caller instead of producing a silent no-op.

Why this matters

#448 (filed today by @nicobailon) reports embed runs that "succeed" with zero embeddings produced. The root cause is exactly the missing precondition: _check_available exists for this purpose but only gets called by EmbeddingStore. Hoisting it into get_provider makes both paths consistent.

Tests: 4 new tests cover both local and embedding-store-with-local paths when _check_available returns False, plus 2 existing tests patched to assert the new precondition path still admits successful constructions. uv run pytest tests/test_embeddings.py -k "TestGetProviderModel or test_local_unavailable or test_embedding_store_unavailable" passes locally.

closes #448

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