Add isolated named Python sessions

[abhishek-anand]

Summary

• add named Python sessions backed by dedicated Jupyter kernels
• support isolated state and concurrent execution through MCP and REST
• discard session kernels on stop and wait for new kernels to become responsive
• stop retrying user-code exceptions while retaining retries for infrastructure failures
• verify MCP 2025-06-18 negotiation and output schemas for every tool

Testing

• session isolation across two kernels
• parallel execution across named sessions
• five-session capacity and overflow behavior
• MCP tool schemas and 2025-06-18 protocol negotiation
• REST session lifecycle and execution
• user-code exceptions execute once and preserve the kernel

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[Copilot]

Pull request overview

This pull request adds named, isolated Python sessions backed by dedicated Jupyter kernels, exposing them via new MCP tools and REST session lifecycle endpoints, and updates retry behavior so user-code exceptions aren’t retried while infrastructure failures still can be.

Changes:

• Introduces PythonSessionManager to create/list/stop named sessions and route execution to session-reserved kernels.
• Adjusts kernel lifecycle handling (discard on session stop, wait for newly created kernels to become responsive, refine retry behavior).
• Adds a contract/integration-style test script covering MCP protocol negotiation/tool schemas plus REST session lifecycle.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
server.py	Adds session manager + MCP/REST session APIs; changes kernel lifecycle + retry/error behavior.
test-sessions.py	Adds end-to-end contract tests for MCP tools, session isolation, concurrency, capacity limits, and REST lifecycle.
README.md	Documents new MCP tools for starting/listing/stopping sessions and how to pass session_id.

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[mkagenius]

I built PR 25's actual head ( 3d60721 ) and ran it. Everything passed: