Fix #7907: Crash when downloading images in Wallet with invalid URL

[StephenHeaps]

Summary of Changes

• Validate image url is a web page url before passing to WebImageDownloader from core. Resolves crashes on iOS until WebImageDownloader will cause a crash if passed url is invalid brave-browser#32419 is resolved.

This pull request fixes #7907

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Test Plan:

1. Open Swap and change network to Polygon
2. Verify no crash when AAVE token is displayed (should be default to token)
3. Open Portfolio and tap Edit User Assets button
4. Optional: change network filters to only have Polygon selected
5. Scroll to bottom of the list
6. Verify no crash when AAVE token is displayed/used

Screenshots:

image.download.crash.resolved.mp4

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).