Summary

• QdrantOps::ensure_collection and ensure_collection_with_quantization previously returned Ok(()) when a collection existed without checking its vector dimensions
• On embedding model change (e.g. 4096-dim qwen3-embedding → 1536-dim text-embedding-3-small), all collections backed by these methods would silently fail at insert/search time with dimension-mismatch errors
• Collections affected: zeph_conversations, zeph_session_summaries, zeph_key_facts, zeph_corrections, zeph_graph_entities, and code-index collections in zeph-index
• Fix: added a private get_collection_vector_size helper using collection_info; both ensure methods now verify dimensions and recreate the collection (with a WARN log) when there is a mismatch

Changes

• crates/zeph-memory/src/qdrant_ops.rs: dimension check + auto-recreation in ensure_collection and ensure_collection_with_quantization; new get_collection_vector_size helper; four #[ignore] integration tests
• CHANGELOG.md: entry in [Unreleased] section

Notes

• EmbeddingRegistry (skills, MCP) already had its own inline dimension detection. That logic remains and still works correctly; it now also benefits from the fix in the underlying QdrantOps call, but the redundancy is harmless.
• Data loss on recreation is intentional and documented in the doc comment and the warning log.

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace --features full -- -D warnings passes
• cargo nextest run --config-file .github/nextest.toml --workspace --features full --lib --bins — 6127 tests pass
• Integration tests (#[ignore]) can be verified manually with a live Qdrant instance at :6334 by running cargo nextest run -p zeph-memory -- --ignored

Closes #1951