AI Chat: don't unload conversations that are associated with non-content-sending pages, e.g. chrome:// or where content is disabled

[petemill]

Resolves brave/brave-browser#42783

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
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• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
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After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[petemill]

I'm not so happy with passing AIChatService everywhere so that AssociatedContentDriver can call service_->OnAssociatedContentDestroyed(content_id) to unload a conversation. Seems a bit messy and difficult to follow and relies on the caller remembering to call that. I thought instead I could make AssociatedContentDelegate have an Observer that AIChatService could observe something like OnConversationsDissassociated on. Seems weird having AssociatedContentDelegate::Observer and AssociatedContentDriver::Observer (*Driver inherits from *Delegate, but I think it's ok in Chromium

[fallaciousreasoning]

is there a reason get_conversation_uuid doesn't return a const std::string&? Seems like we might be doing some unnecessary copying