fix(cli): update test to match new JSON content format

[sanity]

Problem

The message-actions merge changed the CLI's JSON output format for messages. Previously, the message list --json command output:

{"content": {"Public": {"plaintext": "message text"}}}

Now it outputs:

{"content": "message text"}

This is because the CLI now uses effective_text() which returns a plain String (allowing edited message content), rather than serializing the raw RoomMessageBody structure.

The test's decode_plaintext_messages function expected the old nested format, causing the six-peer-regression test in freenet-core CI to fail with "room1-user1 missing expected messages".

Solution

Updated decode_plaintext_messages to extract the content field directly as a string, matching the new JSON format.

Testing

• Verified the test compiles
• This is a test-only change that aligns the test with the new CLI behavior

Impact

Fixes freenet-core CI's six-peer-regression test which is currently failing due to this format mismatch.

[AI-assisted - Claude]