Summary

Closes #6104

Three gaps existed in the plan-mode versioning flow introduced by #6101:

1. FeatureViewPinConflict never raised during feast plan
The conflict check (simultaneous version pin + schema change) only ran inside apply_feature_view(). Users had no way to discover pin conflicts without running feast apply — defeating the purpose of plan mode.

2. Version changes not visible in plan output
diff_registry_objects iterated over spec fields and compared version as a raw proto string ("latest" -> "v2"). The resolved numeric pin target from meta.current_version_number was never surfaced, so users could not see which numeric version they were pinning to or from.

3. No tests for plan-mode versioning
No unit tests covered either of the above.

Changes

BaseRegistry.check_version_pin_conflict() (new method)

Read-only plan-time guard that mirrors the conflict detection inside apply_feature_view. Implemented on BaseRegistry using get_feature_view_by_version and get_any_feature_view — both already declared on BaseRegistry — so it works for both file and SQL registries without duplicating per-registry logic.

Conflict condition: user is simultaneously pinning to an existing version AND the feature view definition differs from the active version (schema or UDF change).

store.plan() — calls the conflict check

Iterates all feature views (FeatureView, StreamFeatureView, OnDemandFeatureView) in desired_repo_contents and calls registry.check_version_pin_conflict() for each, surfacing FeatureViewPinConflict before feast apply runs.

diff_registry_objects() — human-readable version display

• Added "version" to FIELDS_TO_IGNORE (excluded from generic spec field loop)
• Added a dedicated version display block that reads meta.current_version_number for the current state and spec.version for the desired state, producing output like:

  Updated feature_view driver_stats
      version: latest -> v2 (pin)
  
  Updated feature_view orders
      version: v1 (pin) -> latest
  

Test plan

• TestCheckVersionPinConflict::test_latest_version_no_conflict — no-op for latest
• TestCheckVersionPinConflict::test_forward_declaration_no_conflict — pinning to a non-existent version (forward declaration) does not raise
• TestCheckVersionPinConflict::test_pin_no_schema_change_no_conflict — pinning to an existing version without schema change does not raise
• TestCheckVersionPinConflict::test_pin_with_schema_change_raises — pinning to an existing version while also changing schema raises FeatureViewPinConflict
• TestVersionDiffDisplay::test_no_version_change_no_version_diff — identical FVs produce no version property diff
• TestVersionDiffDisplay::test_pin_shows_in_diff — pinning shows "v1 (pin)" in declared value
• TestVersionDiffDisplay::test_unpin_shows_in_diff — unpinning shows "v1 (pin)" in existing value and "latest" in declared
• TestVersionDiffDisplay::test_plan_calls_pin_conflict_check — plan loop calls check_version_pin_conflict for each FV
• ruff format --check and ruff check — clean on all 4 modified files