[toast] Support partial data updates

[chuganzy]

This PR proposes shallow-merging custom toast data when updating a toast.

Currently, the data passed to update replaces the existing data, so fields that should remain unchanged must be passed again:

toastManager.update(id, {
  data: {
    uploadId: '123',
    progress: 50,
  },
});

With this change, the same update can be written as:

toastManager.update(id, {
  data: {
    progress: 50,
  },
});

The existing uploadId is preserved, and data is typed as Partial<Data>, consistent with the other update options.

• I have followed (at least) the PR section of the contributing guide.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5464
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5464
  

commit: ae2ceab

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+40B(+0.01%)	🔺+15B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,129.08 ms -158.17 ms^(-12.3%) | Renders: 76 ⁽⁺⁰⁾ | Paint: 1,783.68 ms -252.05 ms^(-12.4%)

Test	Duration	Renders
Menu open (500 items)	67.77 ms ▼-20.52 ms(-23.2%)	11 (+0)
Combobox type — 500 items, narrows to ~11 (type "Row 25")	32.18 ms ▼-14.10 ms(-30.5%)	15 (+0)
Combobox open — 500 items	33.90 ms ▼-11.96 ms(-26.1%)	4 (+0)
Combobox type — 500 items, all stay mounted (type "Row ")	28.49 ms ▼-7.76 ms(-21.4%)	11 (+0)

11 tests within noise — details

---

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

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