* fix(ui): null-safe trim calls in agent model config (#319)
Guard against undefined values in .trim() calls when editing agent
model configuration (primary model and fallback array entries).
Closes#319
* test: add unit tests for null-safe trim in agent model config
Tests verify that undefined/null values in model primary and fallback
arrays don't cause trim() TypeError crashes.
* test(e2e): add agent model config edge case tests
Tests verify PUT /api/agents/:id with valid config, empty fallbacks,
empty primary, and whitespace-only fallback entries.
nyk