feat(sandbox): add Dockerfile snapshot helper

[Mukil Loganathan (langchain-infra)]

Adds a Python SandboxClient.create_snapshot(..., dockerfile=..., context=...) wrapper for the backend support in langchain-ai/langchainplus#25449.

The SDK flow is intentionally thin:

from langsmith.sandbox import SandboxClient

client = SandboxClient()
snapshot = client.create_snapshot(
    name="my-dockerfile-snapshot",
    dockerfile="Dockerfile",
    context=".",
    fs_capacity_bytes=10 * 1024 * 1024 * 1024,
    on_build_log=print,
)

Under the hood the client creates a temporary builder sandbox, uploads the Docker context, runs docker build --progress=plain with streamed logs, then asks the backend to capture a snapshot from the sandbox-local image.

Release Note

none

Test Plan

• make format
• uv run ruff check langsmith/sandbox/_client.py tests/unit_tests/sandbox/test_client.py
• uv run mypy langsmith/sandbox/_client.py
• TEST=tests/unit_tests/sandbox/test_client.py make tests
• make lint currently fails on pre-existing mypy errors in langsmith/_internal/_orjson.py and langsmith/utils.py; ruff passes.