feat(e2b): add E2B sandbox integration

[vsokolovskii]

Adds E2B as a sandbox provider for Deep Agents Code and ships a new langchain-e2b partner package.

The implementation wraps the E2B SDK in E2BSandbox, wires provider lifecycle into the sandbox factory, exposes the e2b extra through all-sandboxes, and adds CI, release, Harbor, and labeling configuration so the integration is testable and publishable alongside the existing partner packages.

Local validation completed:

• make lock-check
• libs/partners/e2b: ruff check, ruff format --diff, ty check, pytest unit/import tests
• libs/code: ruff check, ruff format --diff, ty check, command catalog check, full unit suite
• libs/evals: ruff format --diff, ruff check --diff, ty check, eval catalog check, unit suite
• Live E2B integrations with E2B_API_KEY: libs/partners/e2b standard sandbox suite passed 86/86; libs/code E2B provider selection passed 8 with 8 expected skips

The evals type check exits successfully but still reports two existing importlib.util warnings in deepagents_evals/radar.py.

[cla-bot]

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