bugfix: LIVE-13226 polkadot send for runtime upgrade

[hzheng-ledger]

✅ Checklist

• npx changeset was attached.
• Covered by automatic tests.
• **Impact of the changes: no impact
  • ...

📝 Description

In react-native, the "app.signWithMetadata" method returns a polkadot signature. It is not a Buffer type variable, we need to cast it to Buffer explicitly. It is only related to Ledger Live mobile app.

❓ Context

• JIRA or GitHub link: https://ledgerhq.atlassian.net/browse/LIVE-13226

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🧐 Checklist for the PR Reviewers

• The code aligns with the requirements described in the linked JIRA or GitHub issue.
• The PR description clearly documents the changes made and explains any technical trade-offs or design decisions.
• There are no undocumented trade-offs, technical debt, or maintainability issues.
• The PR has been tested thoroughly, and any potential edge cases have been considered and handled.
• Any new dependencies have been justified and documented.
• Performance considerations have been taken into account. (changes have been profiled or benchmarked if necessary)

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