[infra] Enable mui/no-floating-cleanup ESLint rule

[romgrk]

Summary

Enables the new type-aware mui/no-floating-cleanup ESLint rule across packages/*/src. The rule (added to code-infra in mui/mui-public#1538) is modeled on @typescript-eslint/no-floating-promises, but for functions: it flags call statements whose return value is a discarded callable (e.g. an unsubscribe / cleanup callback) — a common source of subscription leaks.

subscribe(() => {});            // ✗ discarded unsubscribe — leak
const un = subscribe(() => {}); // ✓
void subscribe(() => {});       // ✓ explicit opt-out

Also flags discards behind unary/logical/conditional wrappers (!subscribe(), a && subscribe(), cond ? subscribe() : x); fluent/builder calls that return this (e.g. d3 scale chaining, incl. inferred this) are not reported.

⚠️ Temporary override — depends on mui-public#1538

The rule isn't released yet, so this branch overrides @mui/internal-code-infra to a local mui-public checkout (the rule's PR):

# pnpm-workspace.yaml
overrides:
  '@mui/internal-code-infra': link:../mui-public/packages/code-infra

This requires mui-public checked out as a sibling ../mui-public on the mui-public#1538 branch, so CI can't install it as-is. Before merge: drop the override and bump @mui/internal-code-infra to a published release containing the rule. The eslint.config.mjs block is the only permanent change.

Leaks found

Running the rule surfaces genuine discarded-cleanup leaks, e.g. MoreEventsPopover.tsx:73 (a subscribeCloseHandler(...) in a useEffect whose unsubscribe is dropped instead of returned as cleanup), plus store subscribe() / plugin-register sites across Data Grid, Scheduler, Tree View, and Chat. The rule is set to error, so the lint job reports these until they're fixed (or it's set to warn) — that surfacing is the point of this PR.

🤖 Generated with Claude Code

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