wpcomsh: maybe_redirect_to_calypso_plugin_pages: quick exit for non-plugin pages

[mreishus]

Proposed changes:

• Lets wpcomsh_maybe_redirect_to_calypso_plugin_pages() skip a few computations if we are not on a page where it can act.
  • This is very minor, but something is better than nothing.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to wp-admin/plugins.php - you should still be redirected to wordpress.com
• Visit other pages and use logging to make sure the quick exit is happening

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Choose a review path based on your changes:
   • A. Team Review: add the "[Status] Needs Team Review" label
     • For most changes, including minor cross-team impacts.
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Wpcomsh plugin:

• Next scheduled release: Atomic deploys happen twice daily on weekdays (p9o2xV-2EN-p2).

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