[core] Move MultistepDialog inline styles to CSS

[dokson]

Summary

Fixes #7710.

MultistepDialog applied min-width: 800px and padding-bottom: 0 via the underlying Dialog's inline style attribute, forcing consumers to use !important to override (as shown in the issue's screenshot).

This moves both defaults into _multistep-dialog.scss under a single .bp6-multistep-dialog class so they can be overridden with normal CSS specificity:

// before (consumer)
.my-dialog {
  /* stylelint-disable-next-line declaration-no-important */
  min-width: 400px !important;
  width: 400px;
}

// after
.my-dialog {
  min-width: 400px;
  width: 400px;
}

The Classes.MULTISTEP_DIALOG (bp6-multistep-dialog) class constant already existed but was never applied to the rendered element; it is now added to the Dialog className.

The user-provided style prop continues to pass through via {...otherProps}, so <MultistepDialog style={{ minWidth: 600 }} /> still works (inline beats class).

Test plan

• pnpm --filter @blueprintjs/core run compile
• pnpm --filter @blueprintjs/core run test:typeCheck
• pnpm --filter @blueprintjs/core run test:vitest:run — 1250 passed
• npx prettier --check on changed files
• Manual smoke check in docs app (deferred to reviewer)

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• Fix (Fixing an issue with existing functionality)
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Description

[core] Move MultistepDialog inline styles to CSS

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[dokson]

@ryannono — this addresses the inline style override issue you reported in #7710. The !important in your screenshot should no longer be needed once this lands.