[github-actions]

The nightly test_types_next job (TypeScript @next) fails while building declarations for @mui/internal-core-docs:

src/sandbox/CodeSandbox.ts: error TS5088: The inferred type of 'createMaterialTemplate' references a type with a cyclic structure which cannot be trivially serialized. A type annotation is necessary.

createMaterialTemplate returns an object whose replaceContent() does return this, so the inferred type contains itself. TypeScript 6 silently turned the recursion into any in the emitted .d.ts. TypeScript 7 asks for an explicit annotation instead. This PR adds a MaterialTemplate return type in CodeSandbox.ts and StackBlitz.ts, so chained replaceContent() calls also stay typed.

The error showed up four times because src/Demo/sandbox was a near-copy of src/sandbox, left over from #48842. The copies had already drifted: the Demo one still used the latest npm tag instead of next and missed the vite pin for stackblitz/webcontainer-core#2104. This PR deletes src/Demo/sandbox, moves MuiChat.ts into src/sandbox, and points Demo at it. As a result, "Open in CodeSandbox/StackBlitz" on demos now gets @mui/material@next and the pinned vite, the same as the templates.

The separate test_types_next TS2883 errors (MUIStyledCommonProps) are not addressed here.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-49497--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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Check out the code infra dashboard for more information about this PR.

[aarongarciah]

Verified locally: with typescript@next (7.1.0-dev) the base commit emits 4× TS5088 and this branch emits none. Core-docs vitest and eslint pass. The deleted Demo/sandbox only differed from sandbox by the next tag and the vite pin that #48842 accidentally reverted, so nothing is lost, and Demo/index.ts keeps the codeSandbox/stackBlitz re-exports.

Non-blocking nits:

• The title undersells the second commit: "Open in CodeSandbox/StackBlitz" on demos now gets @mui/material@next and the pinned vite. Something like "[core-docs] Remove duplicate Demo/sandbox and annotate createMaterialTemplate for TS 7" would read better in the changelog.
• In CodeSandbox.ts, the replaceContent updater is typed to return string, but package.json content is an object and the docs callers return it unchanged, so string | Record<string, any> would be accurate. In StackBlitz.ts files are always strings, so the parameter could just be string. Pre-existing, but the new interface makes it explicit.