feat(e2b): enable `beta_pause()` for e2b sandbox

[gali-leilei]

major changes (potentially breaking):

• upgrade e2b-code-interpreter>=1.2.1 to e2b-code-interpreter==2.0.0. This enables beta_pause().
• downgrade from fastmcp import fastMCP to from mcp.server.fastmcp import fastMCP. This enables mcp dev recipe, as well as notification messages from mcp server.

others small changes:

• uv.lock file update.
• justfile update.
• add max_context_length to config/llm/default.yaml.

the changes were tested in two ways:

1. unit test with just mcp-inspect python_mcp_server.py.

2. e2e test with uv run main.py agent=evaluation llm=default.

Known minor issues.

• http 409 from having timeout parameter. This is a known open issue.
• http 204 from calling beta_pause().
• just mcp-inspect xxx.py still requires setting environment variables.