fix: improve error message for invalid icon imports

[FadhlanR]

Problem

When a card definition has a static icon that is a bad named import (e.g. import { NonExistent } from '@cardstack/boxel-icons/non-existing-path/nope'), JavaScript silently resolves it to undefined. This caused the prerenderer to produce a cryptic [data-prerender] has no child element to capture error, giving developers or ai-bot no actionable information.

Fix

Added a guard in the render/icon route's model() hook that checks whether cardTypeIcon() resolved to a valid component. If not, it throws a descriptive error: static icon is undefined — check that the import resolves to a valid icon component. Because Ember is still intact at this point, the error propagates via the parent render route's error action hook into the render.error route, which renders a [data-prerender-error] element. The prerenderer's captureResult picks this up and surfaces the message in the error document.

Before

After

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Host Test Results

    1 files  ±0      1 suites  ±0   1h 35m 26s ⏱️ - 3m 57s
1 878 tests ±0  1 863 ✅ ±0  15 💤 ±0  0 ❌ ±0 
1 893 runs  ±0  1 878 ✅ ±0  15 💤 ±0  0 ❌ ±0 

Results for commit 0827549. ± Comparison against base commit c2e3fb9.