keep an AI Chat conversation alive if it has chat messages and its associated content is still alive

[petemill]

This aids recall especially whilst the AIChatHistory feature flag is disabled

Resolves brave/brave-browser#42510

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:

[github-actions]

Chromium major version is behind target branch (131.0.6778.85 vs 132.0.6834.15). Please rebase.

[brave-builds]

Released in v1.75.86

[kjozwiak]

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.75.86 Chromium: 132.0.6834.33 (Official Build) nightly (64-bit)
-- | --
Revision | 0e6fd65d499de588288c231ca0aba711876f5022
OS | Windows 11 Version 24H2 (Build 26100.2454)

Using the STR/Cases outlined via brave/brave-browser#42510 (comment), ensured that the conversation session is being saved and not cleared when closing/re-opening the Leo panel as per the following:

Recording.2024-12-10.005905.mp4

As per the conversation with @petemill via https://bravesoftware.slack.com/archives/CHGKGMHDJ/p1733796984842519, Android is running into an issue due to #26951 which is only affecting Nightly, So we'll need to uplift the above into 1.74.x & 1.73.x to verify that the issue has been resolved. If it's still broken, we'll make a decision re: fixing or reverting it from 1.73.x.