Add plan mode support to SQL registry for feature view versioning

## Summary

The proto-based (file) registry supports `feast plan` which shows a dry-run diff of what would change before `feast apply`. The SQL registry does not yet have equivalent plan mode support for the feature view versioning flow.

This includes:
- Showing version changes (pin/revert) in plan output for the SQL registry
- Surfacing `FeatureViewPinConflict` errors during `feast plan` (not just `feast apply`)
- Ensuring the SQL registry's diff logic accounts for version metadata

## Context

Feature view versioning (PR #6101) added version tracking with pin/revert support. The pin conflict detection was added to both the file registry and SQL registry's `apply_feature_view`, but the plan mode path (which previews changes without applying) only works with the file registry today.

## Acceptance criteria

- [ ] `feast plan` correctly previews version pin/revert operations when using the SQL registry
- [ ] `FeatureViewPinConflict` is raised during plan (not just apply) for the SQL registry
- [ ] Plan output shows version metadata changes (e.g., "driver_stats: v1 → v0 (pin)")
- [ ] Tests cover plan mode with SQL registry for versioning scenarios