Fix crash in AI Chat when conversation is unassociated from page content

[petemill]

The associated content wasn't notified in all cases that the conversation was no longer associated with it and would not be notified of destruction. It kept a reference to the conversation and used it after destruction.

Fix brave/brave-browser#42091
Fix brave/brave-browser#42092

Also removes a NOTREACHED_IN_MIGRATION from the same file.
Fix brave/brave-browser#42089

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:

STR in brave/brave-browser#42092

[SergeyZhukovsky]

++

[brave-builds]

Released in v1.75.12

[kjozwiak]

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

Brave | 1.75.13 Chromium: 131.0.6778.39 (Official Build) nightly (64-bit)
-- | --
Revision | 71c03ef2ff1496bd28e5e74715c252941a135b1a
OS | Windows 11 Version 24H2 (Build 26100.2161)

Using 1.75.11 Chromium: 131.0.6778.39 and the STR/Cases outlined via brave/brave-browser#42092 (comment), reproduced the crashes as per the following:

* Crash #1 (Test Case #1) - 46d21800-e4c5-e10c-0000-000000000000
* Crash #2 (Test Case #2) - a5d31800-e4c5-e10c-0000-000000000000

Using the same STR/Cases outlined via brave/brave-browser#42092 (comment) which were used above to reproduce the two crashes, ensured that Brave wasn't crashing with 1.75.13 Chromium: 131.0.6778.39 which has the above fix.

---

Verification PASSED on Pixel 6 running Android 15 using the following build(s):

Brave | 1.75.13 Chromium: 131.0.6778.39 (Official Build) canary (64-bit)
--- | ---
Revision | 71c03ef2ff1496bd28e5e74715c252941a135b1a
OS | Android 15; Build/AP41.240925.009; 35; REL

Using 1.75.11 Chromium: 131.0.6778.39 and the STR/Cases outlined via brave/brave-browser#42092 (comment), reproduced the crashes as per the following:

* Crash #1 (Test Case #1) - e2d61800-e4c5-e10c-0000-000000000000
* Crash #2 (Test Case #2) - 3fd71800-e4c5-e10c-0000-000000000000

Using the same STR/Cases outlined via brave/brave-browser#42092 (comment) which were used above to reproduce the two crashes, ensured that Brave wasn't crashing with 1.75.13 Chromium: 131.0.6778.39 which has the above fix.