feat: Multichain: Migrate getCurrentChainId to use page context

[darkwing]

Description

This PR makes getCurrentChainId return the dapp context chainId when connected to a dapp, falling back to the global chainId when not connected to a dapp.

** TODO **

• Based on this, we should initialize state.metamask.domains to always have metamask entry of whatever the default chainId is a MetaMask install (Mainnet for prod, Sepolia for dev)

Related issues

Fixes: #27832

Manual testing steps

1. Go to this page...

Screenshots/Recordings

Before

After

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.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[Gudahtt]

Worth double-checking why this is undefined, this doesn't seem like it should be possible

[Gudahtt]

At a glance this doesn't seem possible. Maybe caused by a bad fixture.

[NidhiKJha]

I think this is a possibility when the autoswitch toggle is off 🤔