feat: Add MegaETH Testnet as a default test network

[wantedsystem]

Description

Add MegaETH Testnet to default testnet networks

Related issues

Fixes:

Manual testing steps

• Click on show test networks
• You will see Sepolia / Linea Sepolia and MegaETH Testnet
• Click on MegaETH Testnet
• Send a transaction
• Check the transaction result in the explorer

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues
• I've included manual testing steps
• I've included screenshots/recordings if applicable
• 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.
• I’ve properly set the pull request status:
  • [X ] In case it's not yet "ready for review", I've set it to "draft".
  • [] In case it's "ready for review", I've changed it from "draft" to "non-draft".

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.

[wantedsystem]

I have read the CLA Document and I hereby sign the CLA

[metamaskbot]

Builds ready [960fbd0]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
  • content-script: 0
  • offscreen: 0

Page Load Metrics (1703 ± 50 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	1550	1971	1701	100	48
		domContentLoaded	1541	1883	1672	84	40
		load	1550	1983	1703	103	50
		domInteractive	24	80	33	15	7
		backgroundConnect	11	99	36	27	13
		firstReactRender	13	73	35	21	10
		getState	5	84	19	22	10
		initialActions	0	1	0	0	0
		loadScripts	1157	1429	1264	72	35
		setupStore	7	59	19	17	8
		uiStartup	1764	2493	1940	170	82

Bundle size diffs [🚨 Warning! Bundle size has increased!]

• background: 426 Bytes (0.01%)
• ui: 0 Bytes (0.00%)
• common: 705 Bytes (0.01%)