chore: prevent network configuration properties from being modified

[galargh]

• Because this PR includes a bug fix, relevant tests have been included.
• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• I didn't do anything of this.

---

Resolves #5831

As per https://nomicfoundation.slack.com/archives/C03P6B72ZHU/p1732272638933179, this PR ensures that readonly properties referencing NetworkConfig are immutable.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
hardhat	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 28, 2024 11:22am

[changeset-bot]

⚠️ No Changeset found

Latest commit: 484f1ca

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
hardhat	🔄 Building (Inspect)	Visit Preview	💬 Add feedback	Nov 22, 2024 10:38am