Remove unused NetworkRequest.durationDisplay getter and test

[hannah-hyj]

I keep seeing "Dart Code metric" CI failing in PRs that didn't touch [network_model.dart]

This PR is to fix that by removing durationDisplay from NetworkRequest and its corresponding unit test in network_model_test.dart. The getter was not used anywhere in DevTools production code (durations are formatted directly via durationText in DurationColumn and HttpRequestOverviewView), which was causing DCM's check-unused-code to fail with 'getter durationDisplay is used only in tests'.

Pre-launch Checklist

General checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I updated/added relevant documentation (doc comments with ///).

Issues checklist

• I listed at least one issue that this PR fixes in the description above.
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Tests checklist

• I added new tests to check the change I am making...
• OR there is a reason for not adding tests, which I explained in the PR description.

AI-tooling checklist

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• OR I did use AI tooling, and...
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Feature-change checklist

• This PR does not change the DevTools UI or behavior and...
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• OR this PR does change the DevTools UI or behavior and...
  • I added an entry to packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md.
  • I included before/after screenshots and/or a GIF demo of the new UI to my PR description.
  • I ran the DevTools app locally to manually verify my changes.

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[gemini-code-assist]

Code Review

This pull request removes the unused @visibleForTesting getter durationDisplay from the NetworkRequest class in network_model.dart, as well as its corresponding unit tests in network_model_test.dart. There are no review comments, and I have no feedback to provide.