Fix Inspector updates after SubclassSelector type changes

[mackysoft]

Summary

• Fix stale Inspector validation after selecting a new reference type. The popup now reports the edit through the owning field's normal GUI change check, so TriInspector 1.x re-evaluates ValidateInput and dynamic InfoBox messages.
• Store the pending notification in Unity GUI control state so a drawer replacement during the type change cannot lose it.

Scope

• Editor drawer only; no new public API, Runtime dependency, or TriInspector integration layer.
• Exercise real popup input for type changes, null selection, cancellation, and adjacent array element isolation.
• Give Linux EditMode tests an Xvfb display with software OpenGL; GUI interaction tests cannot run with -nographics.

Verification

• Local regression cases: 3 passed on Unity 2021.3.45f2 and 2023.2.22f1. The same cases detect the two missing notifications in the original implementation while cancellation passes.
• Direct Inspector comparison on Unity 6000.3.9f1 with TriInspector 1.15.1: type selection updates ValidateInput, dynamic InfoBox, and OnValueChanged; the standard PropertyField consumer also observes the change. In this reproduction, dirty state and OnValidate already updated before the fix.
• Linux CI: EditMode passed on 2021.3.45f2 (50/50) and 2023.2.22f1 (53/53), including all three new GUI cases. Final Verification passed.
• Independent review completed without actionable findings; workflow YAML and shell syntax checks passed.

References

• The report names package 1.6.1 and Unity 6000.2.6f2, but does not identify a TriInspector version. The asynchronous callback path is present in 1.6.1 and current 1.7.0; the exact reported Unity version was not run.
• TriInspector 1.15.1 checks the built-in drawer with EndChangeCheck and evaluates InfoBox through its validator. Both attributes cache their validation results. Its current 2.0 preview uses a different UI Toolkit update path and was inspected as source only.

Closes #88

[mackysoft]

Pre-merge verification completed for commit 76b43028d7f0becf4fd20eee6921ce0a181f0a63.

Independent review and the final diff check found no actionable issues. Both Linux EditMode jobs pass (Unity 2021.3.45f2: 50/50; Unity 2023.2.22f1: 53/53), and the final Verification check passes. The added popup interaction tests verify the caller-visible change notification; type and null selection fail against the original implementation and pass with this fix.

Merging to main to resolve #88.