Add repl_prepare for Python session initialization

[t-kalinowski]

Summary

• Add a new repl_prepare tool for Python so clients can explicitly warm up and initialize the runtime before sending code.
• This reduces first-call latency and makes session setup behavior more predictable for clients that want to separate preparation from evaluation.
• Expose the new flow through the server, worker lifecycle, backend driver, and Python session/runtime layers.
• Update the public docs and tool descriptions to reflect the new preparation step and the related reset behavior.
• Add coverage for the Python backend and repl_prepare behavior, including the session lifecycle and worker integration paths.

Notes

• Internal changes are concentrated in the Python backend plumbing and session management; the MCP-facing change is the new preparation entry point and its documented contract.
• Completed the associated planning note in docs/plans/completed/python-repl-prepare.md.