test(widgets): TextSubstringHighlighting (+6 tests)

[TaprootFreak]

Summary

Stage 52 of the coverage push. Widget tests for the highlight-substring helper used by legal copy and toast snippets.

Cases

Target

Cases

`TextSubstringHighlighting`

6 — substring-not-found fallback (plain `Text`, internal `TextSpan.children` is null); substring-found splits into three spans (before / match / after); default `highlightedStyle` is `base.copyWith(fontWeight: bold)`; custom `highlightedStyle` replaces the default bold entirely; `onHighlightedTap` attaches a `TapGestureRecognizer` and firing it triggers the callback; null callback → recognizer is null

What's pinned

• The fallback vs highlight branch is observable from the rendered tree: the fallback wraps a plain `Text` (internal `RichText.text.children == null`), the highlight branch builds a `RichText` directly with three children. Pin both.
• The default highlighting style preserves the base font size (`copyWith`) — refactoring the default to a fresh `TextStyle()` would break callers that rely on font-size inheritance.
• The tap recognizer is optional: when `onHighlightedTap` is null, no recognizer is attached so the span doesn't intercept touches.

Test plan

• `flutter test test/widgets/text_substring_highlighting_test.dart` — 6 pass
• `flutter analyze` clean on the new file
• CI green