chore(coinbase): migrate Buy endpoints from v2 to v3

[Syn-McJ]

Issue being fixed or feature implemented

Coinbase is going to shut down some v2 API endpoints. We need to migrate, starting from the Buy feature

What was done?

• Replace v2/accounts/{account_id}/buys with api/v3/brokerage/orders calls.
• Rearrange Buy flow: no need to create and commit an order as the /orders endpoint executes the order immediately.
• Improve HTTPClient errors.
• Fix the de-authentication button.

How Has This Been Tested?

QA

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone