Prevent CLI pushes from changing existing page path aliases

Overview

Canvas page pushes currently allow a local page file to update the path alias of an existing remote page when the UUID matches. This can unintentionally change the public URL of an existing page during a content push, and/or cause other issues.

Steps to reproduce:

1. Pull or create a local page file with a UUID matching an existing remote page.
2. Change the local page file's path value to a different alias.
3. Run canvas push --include-pages.
4. The existing remote page can be updated with the new alias.

Proposed resolution

Add remote-aware page validation during canvas push so existing pages are checked against their remote path aliases before pushing.

If a local page UUID matches an existing remote page and the normalized local path differs from the normalized remote path, report a validation error and stop the push.