fix: duplicate network validation in 12.3

[bergeron]

Description

Fixes issue 27456 in 12.3.

If you try to add a new network with the same RPC URL and chain id as an existing network, the link presented in the error message does not work and throws an error:

Screen.Recording.2024-09-27.at.11.43.41.AM.mov

This link in the error message was intended for the network redesign and should stay behind the feature flag. Duplicate chain IDs are allowed in this version of network managment. So this PR makes it back into a warning like it would have been in 12.2:

There's nothing to fix in develop since it is already on the new network controller / UI

Related issues

Fixes: #27456

Manual testing steps

1. Try to add a new network with the same RPC URL and chain id as an existing network.

Screenshots/Recordings

Before

After

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Pre-merge reviewer checklist

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[Gudahtt]

I had a bit of trouble at first following how this change was related to the error. But I see that the UI with the bug is only shown when this existingChainId error is triggered, which is prevented by this change.

Makes sense! LGTM

[metamaskbot]

Builds ready [a7fd3f5]

• builds: chrome, firefox
• builds (beta): chrome
• builds (flask): chrome, firefox
• builds (MMI): chrome, firefox
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Background Module Init Stats
• mv3: UI Init Stats
• mv3: Module Load Stats
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• code coverage: Report
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

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

Page Load Metrics (1870 ± 173 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	1649	3399	1874	359	172
		domContentLoaded	1599	3296	1812	350	168
		load	1640	3399	1870	361	173
		domInteractive	12	83	38	17	8