Add `BackendIntegrationTests` for Deep Agents `BackendProtocol` implementations

### Submission checklist - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [x] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description `langchain-tests` currently provides `SandboxIntegrationTests` for Deep Agents `SandboxBackendProtocol` implementations, but there does not appear to be an equivalent standard test suite for plain `BackendProtocol` implementations. I’d like to propose adding a `BackendIntegrationTests` suite for storage/filesystem backends that implement: - `read` / `aread` - `write` / `awrite` - `edit` / `aedit` - `ls` / `als` - `glob` / `aglob` - `grep` / `agrep` - `upload_files` / `aupload_files` - `download_files` / `adownload_files` This would give backend integration authors a standard compatibility suite without requiring sandbox execution support. ### Use Case This would help developers building Deep Agents backend integrations for storage systems such as Azure Blob Storage, S3, Postgres, local/remote filesystems, or other virtual filesystem implementations. These backends may implement `BackendProtocol` fully but may not support `SandboxBackendProtocol.execute()`. A backend-only standa…