Update ModalContent for pure-black elevated surface

[georgewrmarshall]

Description

Update ModalContent to render the correct elevated dialog surface in pure-black dark mode once PureBlackProvider is available.

Today the dialog uses BoxBackgroundColor.BackgroundDefault, which matches the page background in standard dark mode but does not provide the elevated surface hierarchy required in pure-black mode.

Depends on

• Add PureBlackProvider and usePureBlack to @metamask/design-system-react #1275 Add PureBlackProvider and usePureBlack to @metamask/design-system-react
• Add pureBlackDarkTheme to @metamask/design-tokens #1272 Add pureBlackDarkTheme to @metamask/design-tokens (for elevated surface tokens)

Acceptance Criteria

• ModalContent uses token-backed elevated surface styling in pure-black dark mode
• Standard grey dark mode appearance is unchanged when isPureBlack is false
• No consumer-level pure-black conditionals are required at call sites
• Storybook coverage demonstrates standard dark vs pure-black modal hierarchy
• Component tests cover pure-black branching where practical

Constraint

• isPureBlack=false uses the existing grey dark theme
• isPureBlack=true uses pureBlackDarkTheme

References

• Epic: Epic: Pure Black Theme and Provider #1271
• Related audit: Adopt pure-black surface elevation behavior in MMDS components #1276
• Jira epic: TMCU-622