fix: 9142 search results disappearing

[numericOverflow]

Summary

When a remote search saves an envelope-only message and a subsequent sync replaces it with the full message, the database.replace() call deletes and re-inserts the row. This orphans the corresponding threads entry (foreign key on messages.id), causing the message to disappear from search results.

Switching from replace() to update() preserves the row ID and keeps the thread reference intact.

Issue

Fixes #9142

Test plan

• Added test: replace envelope message with full message verifies thread entry preserved
• Existing tests pass (SaveMessageOperationsTest)

AI disclosure

AI was used to assist in tracking down source of bug, correcting & drafting this pull request description. It wasn't a simple vibe code job, I did interrogate it on plan and helped it choose the most appropriate, reliable and logical solution.

I tested the resulting build on my android device and confirmed it fixed the bug that has been outstanding for several years.

Supporting information:

Report from Claude 4.6 (High)
root-cause-analysis.md

Solution that was implemented after discussion and design choices from Claude 4.6 (High)
final-solution-proposal.md

report: include

[github-actions]

✅ Validation Passed: All report and feature-flag labels are correctly set.