fix: replace fraxTestnet with fraxSepolia

[alex-miao]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
• Did you run lint (yarn lint:check) and fix any issues? (yarn lint:write)
• Did you follow the contribution guidelines?

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PR-Codex overview

The focus of this PR is to replace occurrences of fraxtalTestnet with fraxtalSepolia across multiple files.

Detailed summary

• Replaced fraxtalTestnet with fraxtalSepolia in various files
• Updated chain configurations and addresses accordingly

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