zulip: Add remove_storage helper to Client.

[hanyucrocks]

Mirrors update_storage and get_storage, sending DELETE to /bot_storage. With no argument, removes all data stored for the bot; passing {"keys": [...]} removes only the specified entries.

The corresponding endpoint is bot-only as of Zulip 12.0 (API feature level 494), so this helper only succeeds for bot users.

Fixes #637.

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.

[chrisbobbe]

It looks like you wrote documentation for this, in zulip/zulip#38786 . Thanks for that!

I think a code comment linking to https://zulip.com/api/remove-bot-storage would be helpful here, but (from a quick look) it looks like that's not been our practice in this code. Is @amanagr the right person to confirm that we want to start doing that?