Add isMandatorySetter API to eliminate string checking

- Add computed.isMandatorySetter() API to replace string checking for 'You attempted to update'
- Update object-inspector.js to use new API instead of checking setter code
- Update EMBER_INSPECTOR_API_ANALYSIS.md with new API definition

This eliminates the last instance of checking internal implementation details via string matching.

Author: patricklx

.bob/notes/pending-notes.txt

@@ -0,0 +1,3 @@
+{"id":"4e60dc2f-cb5b-4c9f-a694-d6808db94ca3","ts":"2026-02-27T10:28:02.624Z","path":"/Users/patrickpircher/IdeaProjects/ember-inspector/ember_debug/utils/ember-inspector-api.js","version":"1.0.0","taskID":"6d3c353f-c5a3-4f61-a387-a8e50b1c2c7d"}
+{"id":"46e77bf6-ba36-4887-9038-4aa94eb76e2f","ts":"2026-02-27T10:28:22.657Z","path":"/Users/patrickpircher/IdeaProjects/ember-inspector/ember_debug/object-inspector.js","version":"1.0.0","taskID":"6d3c353f-c5a3-4f61-a387-a8e50b1c2c7d"}
+{"id":"3f0443dc-64df-4c72-881b-a544995b3162","ts":"2026-02-27T10:29:06.616Z","path":"/Users/patrickpircher/IdeaProjects/ember-inspector/EMBER_INSPECTOR_API_ANALYSIS.md","version":"1.0.0","taskID":"6d3c353f-c5a3-4f61-a387-a8e50b1c2c7d"}