Sort unread topics by recency for n shortcut.

[archie-2006]

Fixes #1582.

What does this PR do, and why?

Before, using the n keybinding (NEXT_UNREAD_TOPIC) jumps to the next unread topic alphabetically within a stream. But it should be sorting unread topics by recency.

this pull request fixes this issue #1582 by using recency-based sorting

1. helper.py: Updated sort_unread_topics to accept an optional topic_msg_ids dictionary. Modified the sorting key to evaluate the maximum message_id within each topic (-max(...)), ensuring that the most recently active topics go to the top (bubble). The alphabetical topic name sorting (from earlier) is still being used as a secondary tie breaker.
2. model.py: Updated calls to sort_unread_topics in next_unread_topic_from_message_id to pass self.index["topic_msg_ids"]

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes Using n should move between topics by recency #1582
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual Changes

Navigating with n in Explore mode. The selection correctly jumps from the recently active (checkmark) test topic to the next most recent unread topic (test), rather than sorting alphabetically, like earlier.

The header shows being in the test checkmark in the first screenshot (this is before pressing n)

In the second ss, after pressing n, the ui changes and the header now shows test without the checkmark and the message history of unread messages for that topic beneath it.