* feat: migrate LangSmith sandbox creation to snapshot API
Replaces the template-based sandbox flow (DEFAULT_SANDBOX_TEMPLATE_NAME /
DEFAULT_SANDBOX_TEMPLATE_IMAGE) with the new snapshot-based flow.
- New required env var DEFAULT_SANDBOX_SNAPSHOT_ID (UUID of a pre-built
LangSmith snapshot; build out-of-band via UI or SandboxClient.create_snapshot)
- Optional DEFAULT_SANDBOX_SNAPSHOT_FS_CAPACITY_BYTES overrides the root FS
size at boot (default 32 GiB)
- Startup-time validation via a FastAPI lifespan hook: the server refuses
to boot with a clear ValueError if SANDBOX_TYPE=langsmith and
DEFAULT_SANDBOX_SNAPSHOT_ID is unset, so failures surface in boot logs
rather than on the first thread
- Reconnect-to-existing-sandbox path unchanged
- Docs (INSTALLATION.md, CUSTOMIZATION.md) updated to describe the new
snapshot workflow
* fix: format create_sandbox_snapshot.py to pass ruff
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Co-authored-by: aran-yogesh <yogesh.mahendran@langchain.dev>
