fix(desktop): avoid drafts from persistent agent hydration

[MajorTal]

Hey good people, I'm Tal and this is my first contribution to this (awesome) project. It's a bug that was bothering me while dogfooding Buzz. I hope it's in good order...

Summary

Opening a thread with Keep addressed agents active enabled hydrated the composer with programmatic agent mentions. When the thread unmounted, the normal draft lifecycle serialized those mentions as if the user had authored them, creating an Inbox draft without any user input.

This change records the exact editor state produced by persistent-agent hydration and resolves only that unchanged state to empty content when persisting a draft. Any user edit makes the content differ and is preserved normally. Existing mention-only drafts, attachment-only drafts, thread switches, and the post-send recipient state are covered by regression tests.

User impact: viewing a thread no longer creates a ghost draft, while real unsent content and attachments continue to be saved.

Related issue

Fixes #3376.

No duplicate open PRs were found for this issue.

Testing

• . ./bin/activate-hermit && just ci
• 10 focused StrictMode production-hook tests in MessageComposerDraftImagePersist.test.mjs
• 8/8 Chromium tests in persistent-agent-audience.spec.ts, including the full open-thread/leave-thread draft regression
• Manual local UI smoke test: enabled persistent agents, selected a managed agent, and sent a message through the composer

Manual verification:

1. Enable Settings → Keep addressed agents active.
2. Open a thread without typing, then return to Inbox.
3. Verify the thread does not appear as a draft.
4. Enter unsent content or add an attachment, leave the thread, and verify the authored draft is retained.

There is no visual or layout change, so a before/after screenshot would be identical; this PR changes only draft-persistence behavior.

[Chessing234]

good catch on hydration drafts. does the same path fire for persona reloads after reconnect, or only the initial managed-agent hydrate?

[MajorTal]

Good question! The fix covers the exact composer state created by persistent-agent hydration, including after sending. A persona reload on the same thread does not trigger hydration by itself, but if reconnecting remounts the composer, it goes through the same protected path. I do not have a reconnect-specific test yet, so happy to add one if there is another path I missed.