feat(canvas): gracefully filter excluded fields based on metrics security policy

[burnerlee]

When a metrics view security policy excludes a measure or dimension, canvas components now silently drop those fields instead of showing a ComponentError or a stuck loading/floating UI.

• KPI Grid: excluded measures are filtered from the grid before rendering — only accessible KPIs are shown
• Table (flat): inaccessible columns are silently dropped; only errors if no columns remain
• Pivot: inaccessible measures/dimensions are filtered out; only errors if nothing usable remains
• Leaderboard: same filtering approach as table
• Pivot display now passes isLoading state to the schema validator to prevent a brief "Metrics view not found" error on initial load

Closes 8850

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!