[vector_graphics] Provide textDirection for semantics label to avoid crash without Directionality

[mbcorona]

Description

When a VectorGraphic (and therefore SvgPicture, which delegates to it) is given a semanticsLabel but there is no Directionality ancestor in the widget tree, the app crashes at frame flush with:

A SemanticsData object with label "..." had a null textDirection.
package:flutter/src/semantics/semantics.dart: Failed assertion: 'attributedLabel.string == '' || textDirection != null'

The Semantics node created for the label in _VectorGraphicWidgetState.build passed a non-empty label but never supplied a textDirection, and did not resolve the ambient Directionality. With no Directionality in scope, the null text direction propagated to SemanticsData, which asserts. (This is why the issue only reproduces outside of a MaterialApp/WidgetsApp, where a Directionality is normally present — a triager was previously unable to reproduce it for that reason.)

This PR resolves the text direction with Directionality.maybeOf(context) ?? TextDirection.ltr, so a labeled decorative graphic no longer crashes when no Directionality is in scope.

The pre-existing test Can add semantic label had to wrap the widget in a Directionality to pass; this PR adds a regression test that omits the wrapper and verifies no exception is thrown and the label is present.

Issues fixed by this PR

Fixes flutter/flutter#175532

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[gemini-code-assist]

Code Review

This pull request bumps the vector_graphics package version to 1.2.3 and fixes a crash that occurs when a semanticsLabel is provided without an ambient Directionality by fallback-initializing textDirection to TextDirection.ltr. A corresponding widget test has been added to verify this behavior. The review feedback suggests reusing the existing textDirection state variable in the build method instead of performing another lookup with Directionality.maybeOf(context).