Standardize atomic upsert semantics and error handling across vector store adapters

### Description
This issue improves vector store adapter reliability and safety in `pydebeziumai`. It standardizes atomic upsert semantics across all adapters to prevent data loss during transient embedding failures, improves `MilvusAdapter` error handling and connection management, and clarifies adapter interface contracts.

1. **Atomic Vector Upserts & Data Loss Prevention**:
- Update `ChromaAdapter.upsert` and `MilvusAdapter.upsert` to eliminate non-atomic `delete(id)` + `add_documents(...)` execution steps.
- Use native atomic upserts or direct `add_documents` calls to prevent documents from being deleted if subsequent embedding calls fail.
2. **Milvus Adapter Resiliency & Connection Safety**:
- Catch missing collection `MilvusException` (`code == 100`) inside `MilvusAdapter.delete()` and handle it gracefully as a no-op.
- Hash `connection_uri` into Milvus connection aliases (`f"pydebeziumai_{hash(uri)}"`) to prevent multi-instance alias collisions.
- Add strict type assertions in `_dict_to_milvus_expr` to prevent string format expression fallthroughs.
3. **Interface Contracts & Docstring Accuracy**:
- Update `VectorStoreAdapter` base class docstring requiring single atomic upserts.
- Update `PGVectorAdapter` docstring clarifying sync and async `postgresql+psycopg://` URL compatibility.