Scaffolding for canvas component validation in the runtime

[begelundmuller]

This PR adds scaffolding for validating canvas component properties on the backend. This enables us to give AI agents better feedback when they develop canvas components.

Next steps not handled in this PR:

• Validation can now be implemented in runtime/canvas/component.go and tested in runtime/canvas/component_test.go
• We may want to refactor the runtime/ai/create_chart.go file to use the runtime/canvas package for validation

Closes https://linear.app/rilldata/issue/PLAT-451/scaffolding-for-canvas-validation-on-the-backend

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!

[pjain1]

is vega_lite a renderer? If yes then should be added otherwise previously valid canvas will now have error.

[djbarnwal]

vega_lite is not a valid component renderer. We used to have it in our initial version of canvas. It's not used anymore.

[begelundmuller]

@pjain1 I understand the confusion, here's a PR to remove the old code: #9179