integrations/clickup: Add ClickUp integration script.

[sathwikshetty33]

This is a replacement PR for #824 .

The original PR used an OAuth approach. The bot was created without a ClickUp token, and the script ran an OAuth flow to generate one. That token, along with the team ID, was then appended to the bot's webhook URL as query parameters, so the key essentially lived in the URL.

The server side has since moved on. It now reads the ClickUp token from the bot's config data, where it is a required option at bot creation, and uses it to call ClickUp's API and turn the entity IDs in webhook payloads into readable names.

That is what rules OAuth out. The config option needs the token when the bot is created, but an OAuth flow can only return a token after the bot and its webhook URL already exist, so there is no point at which OAuth could provide what bot creation asks for.

This PR therefore drops OAuth entirely. Instead, the user generates a personal API token from the ClickUp UI (Settings > ClickUp API > API Token), enters it when creating the bot, and passes the same token to this script to register the webhook. The script now registers a plain webhook URL with nothing appended, since the server takes the team ID straight from the payload and the token from config.

Fixes part of zulip/zulip#26529
CZO: thread

How did you test this PR?
Manually invoked the script and added unit tests.
Testing video

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).
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Completed manual review and testing of the following:

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[sathwikshetty33]

@0xthedance tagging you for a review.

[sathwikshetty33]

@zulipbot add "buddy review"