Update AI Governance settings page to use new addon licensing

## Context

PR https://github.com/coder/coder/pull/21598 added a new AI Governance settings page at `/deployment/ai-governance` that displays AI Bridge configuration options.

Once https://github.com/coder/coder/pull/21499 lands (which introduces the AI Governance addon licensing model), we should update the AI Governance page to properly integrate with the new licensing.

## Tasks

- [ ] Update the AI Governance page to check for the AI Governance addon entitlement instead of just `aibridge.enabled`
- [ ] Consider showing different UI states based on:
- No AI Governance addon → Show Paywall/upsell
- AI Governance addon but AI Bridge not enabled → Show config options with guidance to enable
- AI Governance addon with AI Bridge enabled → Show full config options
- [ ] Potentially include Boundary-related settings in the AI Governance page (since AI Governance addon includes both Bridge and Boundary features per #21499)
- [ ] Update sidebar visibility logic if needed based on addon presence

## Related

- PR #21598 - feat(site): move AI Bridge settings to AI Governance page
- PR #21499 - feat: extend premium license for `aigovernance`
- Issue #1246 - Update licenses page to mention new verbiage around AI Governance SKU

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Created on behalf of @dannykopping