[BYOM] Moves page-content to own entry

[jonathansampson]

By moving the page_content to its own conversation entry, we prevent the chat history within Leo from being inundated with distilled content for BYOM users. This approach was verified to work with both a local Ollama instance, and also with ChatGPT.

Resolves brave/brave-browser#41079

Left: Currently Nightly showing backfilled page content into chat history.
Right: Local build with suggested changes showing no backfilling.

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
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• 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
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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:

1. Visit https://sampson.codes/ua
2. Open Leo, and select a custom model
3. Click "Summarize this page"
4. Verify that the page summary is not added to the visible prompt

[jonathansampson]

Rebasing to resolve chromium-version-mismatch.

[LorenzoMinto]

@jonathansampson does it work also for custom messages (non-summary) while using the page context?

[jonathansampson]

@jonathansampson does it work also for custom messages (non-summary) while using the page context?

Sorry, can you elaborate on the scenario? Not sure I fully understand.

[LorenzoMinto]

Instead of clicking Summarise this page you ask a free-text question in the input box

[jonathansampson]

Instead of clicking Summarise this page you ask a free-text question in the input box

Yes, regular chat messages also work when page content is included.

[brave-builds]

Released in v1.73.23