Fix #6965: Duplicate add network requests can be shown

[StephenHeaps]

Summary of Changes

• Adding a new ethereum chain requires a network call, however we were not waiting for the response of attempting to add the new chain before dismissing the Add Network request modal. This could create a race condition where we still have the add network request pending when we are dismissing the Add Network request modal, so it would be shown again.
• Added a loading state and error alert to SuggestedNetworkView. Error will dismiss the modal after as the add chain request is removed in core: https://github.com/brave/brave-core/blob/b31c48fff4b9aecdef95a9bba344f9329b3ab1e7/components/brave_wallet/browser/json_rpc_service.cc#L521
• Added an optional completion closure to handleWebpageRequestResponse so SuggestedNetworkView knows when to start/stop loading, and can now show an error alert when failing to add the network

This pull request fixes #6965

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Test Plan:

1. Visit Chainlist.org
2. Connect your wallet and try adding a few chains.
3. Verify each only shows 'add network' modal once before showing 'switch network' modal.
4. Find Astar on Chainlist.org and try to add it.
5. (assuming it still fails) Verify loading state is shown, and wait for timeout on the request.
6. Verify closing the alert will dismiss the Add Network modal as the request is removed when error is received.

Screenshots:

Add Network Fixed:

add.network.request.mov

Add Network error alert (new):

add.chain.timeout.mov

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).