Reject invalid notebook insert cell kinds

[jlewi]

Summary

• validate every notebooks.update insert cell kind before mutating the notebook
• reject values other than code or markup with an actionable error
• add a regression test proving a later invalid cell prevents earlier valid cells from being partially inserted

Why

The TypeScript type is not enforced for JavaScript executed through AppKernel. Previously, any value other than markup silently fell through to a code cell, so kind: markdown created the wrong cell type instead of failing.

Validation

• pnpm -C app exec vitest run src/lib/runtime/runmeConsole.test.ts
• runme run build test
• Prettier check for both changed files

The repository-wide app typecheck was also attempted and remains red on unrelated existing errors outside this change.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate insert kinds before applying any operation

When a notebooks.update request contains an earlier valid mutation followed by a later insert with an invalid kind (for example operations: [update, insert(kind: "markdown")]), this validation only runs once the insert operation is reached, after the earlier operation has already mutated the notebook in the update loop. The API then rejects but leaves a partial change behind, which defeats the new “reject before mutating” behavior for multi-operation updates; pre-validate all insert specs before applying any operation or roll back the whole update on this error.

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