[Gusto R3] Add Gusto workflow helpers and final-approver fallback logic

[yuwenmemon]

Add utility helpers that the Workflows page needs to determine Gusto's approval mode and correctly fall back to the Gusto final approver in Basic/Manager modes.

Parent Issues: Expensify/App#85850, Expensify/Expensify#591910
Design Doc Section: Detailed — Add Gusto workflow helpers and final-approver fallback logic

Files to add/update

• src/libs/PolicyUtils.ts — Add helpers like getGustoApprovalMode(), isGustoCustomMode(), and isGustoReadOnlyWorkflowMode().
• src/libs/WorkflowUtils.ts — Make sure workflow generation falls back to Gusto's finalApprover in Basic/Manager modes if the backend does not also mirror that value into policy.approver.

Key implementation details

• Treat Gusto Basic and Manager as read-only workflow modes.
• Treat Gusto Custom as "workflow editing still allowed, but approval mode lives on the HR page."
• If the policy is in a non-custom Gusto mode and policy.connections.gusto.config.finalApprover is set, use that as the default approver for any chain-building logic that currently falls back to policy.approver.

Issue Owner

Current Issue Owner: @shubham1206agra

[melvin-bot]

Triggered auto assignment to Design team member for new feature review - @dannymcclain (NewFeature)