Add langchain-sprites partner package (Fly.io Sprites sandbox backend)

[kylemclaren]

Feature

Add a langchain-sprites partner package under libs/partners/sprites. The package is a sandbox backend for Fly.io Sprites. It implements the SandboxBackendProtocol through the sprites-py SDK.

Motivation

Sprites are persistent Linux VMs with names. A Sprite starts in millisconds. A Sprite stops automatically when it is idle, and a stopped Sprite has no cost. A Sprite can make a checkpoint of the full machine state in less than one second and can restore it. These properties are a good fit for Deep Agents: an agent can keep its environment between runs, and a user can roll back the changes an agent makes.

The JavaScript version of this provider is already open: langchain-ai/deepagentsjs#807.

Proposal

• SpritesSandbox extends BaseSandbox: execute() with bash -lc, upload_files() and download_files() through the Sprites filesystem API.
• A .sprite property for direct SDK access (checkpoint, restore, reconnect by name).
• Unit tests with a mock SDK, and the SandboxIntegrationTests standard suite as integration tests. The suite passes 79 of 86 tests against the live Sprites API; the 7 failures also occur for the first-party LocalShellBackend (released langchain-tests expects older glob()/write() semantics).
• Repository wiring per libs/partners/AGENTS.md.

A PR is ready: #6001.

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