Start the kernel on demand when executing scratchpad code

[manzt]

Running scratchpad code—for example from the executeCode tool an agent uses
to drive a notebook—previously required a kernel session to already be running.
If the notebook was open but no cell had been run yet, the request quietly went
nowhere and the caller was left waiting on a result that never arrived.

Start the kernel on demand instead, the same way running a cell does: resolve
the interpreter for the notebook's selected kernel and bring up a session if one
isn't already running. An agent can now act on a notebook from the very first
step rather than depending on the user to run a cell first.

When no kernel is selected at all, that's reported back as an actionable signal
rather than failing silently, so the caller can ask the user to pick one.

[Copilot]

Pull request overview

This PR updates scratchpad execution so the server can create a kernel session on-demand (instead of requiring a pre-existing notebook run), and propagates the selected Python executable through the scratchpad API so the correct environment can be used consistently across extension + server.

Changes:

• Make execute-scratchpad session-scoped by including an executable and creating/recreating the server session on demand when missing (or when the executable changes).
• Refactor sandbox controller logic to expose a reusable “resolve interpreter for this notebook” effect, and route scratchpad runs through the active controller’s resolved executable.
• Add coverage to ensure scratchpad execution does not hang when no session exists yet.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_server.py	Adds executable to scratchpad requests and adds a regression test that scratchpad creates a session when missing.
src/marimo_lsp/api.py	Changes scratchpad handler to accept SessionCommand and create (or recreate) a session when needed.
extension/src/types.ts	Makes execute-scratchpad use SessionScoped<> (requires executable).
extension/src/kernel/SandboxController.ts	Extracts PEP-723 env sync + interpreter resolution into resolveExecutable.
extension/src/kernel/NotebookControllerFactory.ts	Documents the PythonController.executable field.
extension/src/kernel/KernelManager.ts	Resolves an executable from the active controller and includes it in scratchpad commands; introduces NoActiveKernelError.
extension/src/kernel/ControllerRegistry.ts	Adds resolveControllerExecutable(...) helper for Python vs Sandbox controllers.
extension/src/features/RegisterLanguageModelTools.ts	Improves user-facing guidance and catches NoActiveKernelError for LM tool execution.

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[github-actions]

Coverage Report

Area	Lines	Branches	Functions
Python	50.15% (545 / 1030)	39.55% (106 / 268)	—
TypeScript	48.95% (2364 / 4829)	40.47% (937 / 2315)	45.91% (646 / 1407)

_{TypeScript statements: 48.77% (2470 / 5064)}