[vector_graphics] Provide textDirection for semantics label to avoid crash without Directionality (#11962)

## 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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