Mark edits as read immediately.

[dgmltn]

fixes #15021

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Pixel 9, Android 16
• Pixel 10 Pro, Android 17

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

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Description

What this fixes

An incoming edit is stored with the original message's date_received, so it never advances the conversation's mark-read watermark and is never marked read while you're looking at it, the sender keeps seeing the edit as unread until they send another message. This marks the new revision read in EditMessageProcessor right after a successful insert, (only when the edit's thread is the one on screen).

Testing

Added EditMessageProcessorTest_markRead (instrumented, 3 cases), and confirmed it catches the bug: with the fix reverted the visible-thread case fails, with it applied all three pass. Run on a Pixel 9.

Manually tested the bug on main and on this branch. Before the fix, the read message never appeared as read, until a later message was sent (or the receiver exited to the list screen and reopened the conversation screen). After the fix, the message was marked as read when expected.