fix: canvas sub/unsub race condition

[AdityaHegde]

In #9606 we fixed canvas sub/unsub issues when moving from a canvas route to non-canvas route or a different canvas route. But going from canvas to same canvas route, (the preview button in rill dev) still has the old issue of components being unmounted.

Improving the checks to have a count of subsriptions per canvas. This way mounting and unmounting the same canvas doesnt leave the stores in unsubbed state.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!