rss-bot: handle oldest-first RSS feeds correctly.

[joshishiv4]

Made changes to avoid stopping early when encountered previously seen entries so newer items later in the feed are not missed.

Fixes: #831

As discussed on CZO, some RSS feeds (such as uptime.com) are ordered oldest-first rather than newest-first. The RSS bot previously assumed reverse chronological ordering and could stop scanning early, causing newer entries later in the feed to be missed.

This change updates the scanning logic so the bot continues iterating through the feed, ensuring newer entries are detected regardless of feed ordering.

How did you test this PR?

• Tested locally using an oldest-first RSS feed:
  https://status.uptime.com/statuspage/uptime-status/subscribe/rss
• Verified behavior using the #test here stream
• First run posted 3 initial entries
• Second run posted 7 additional newer entries that were previously missed
• At the third run we had no new feed as all old feed was scanned and stores in hash

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

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Recreated from zulip/python-zulip-api#894