fix: Fix currentNetwork selector when current network config is missing (#26327)

[Gudahtt]

This is a cherry-pick of #26327 for v12.0.1. Original description below:

Description

The providerConfig state is not guaranteed to match a built-in or custom network. It should in the vast majority of cases, but there are some edge cases where that would not hold true. For example:

• If the wallet crashed partway through removing the current network
• If the user has had the same network selected since before we introduced the networkConfigurations state

The currentNetwork selector has been updated to handle this case by returning a configuration object constructed from the providerConfig state directly.

Related issues

Fixes #26320

Manual testing steps

• Create a dev build and proceed through onboarding
• Run this command in the background console to set providerConfig to a custom network that isn't present in the user's tracked network configurations:

  chrome.storage.local.get(
    null,
    (state) => {
      state.data.NetworkController.providerConfig = {
        rpcUrl: 'https://mainnet.optimism.io',
        chainId: '0xa',
        ticker: 'ETH',
        type: 'rpc',
      };
      chrome.storage.local.set(state, () => chrome.runtime.reload());
    }
  );
  

• See that the UI can be opened and does not crash.

Screenshots/Recordings

N/A

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding
  Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.66%. Comparing base (219dca5) to head (0b62420).

Files	Patch %	Lines
ui/selectors/selectors.js	37.50%	5 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           Version-v12.0.1   #26331   +/-   ##
================================================
  Coverage            65.66%   65.66%           
================================================
  Files                 1369     1369           
  Lines                54661    54663    +2     
  Branches             14223    14225    +2     
================================================
+ Hits                 35890    35893    +3     
+ Misses               18771    18770    -1     

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[metamaskbot]

Builds ready [0b62420]

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Page Load Metrics (45 ± 5 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	65	135	79	18	9
		domContentLoaded	8	18	10	2	1
		load	39	85	45	10	5
		domInteractive	8	18	10	2	1