fix(e2b): remove sandbox lifecycle provider

[vsokolovskii]

Overview

Removes E2BProvider from the public package API so langchain-e2b mirrors the other sandbox integrations: users create/manage the sandbox with the provider SDK and pass the connected sandbox to E2BSandbox.

This keeps the package focused on adapting an E2B sandbox to the Deep Agents sandbox protocol and avoids duplicating lifecycle behavior already owned by the E2B SDK.

Changes

• deletes langchain_e2b.provider and its unit tests
• removes E2BProvider from langchain_e2b.__all__
• restores the README quickstart to Sandbox.create() + E2BSandbox(...)
• adds an export guard test so E2BProvider is not reintroduced accidentally
• bumps package metadata and lockfile to 0.0.3
• updates the changelog for 0.0.2 and 0.0.3

Validation

• make test
• make lint
• uv build --no-sources
• inspected the built wheel to confirm provider.py is not included