test: add tests for composables

[rijkvanzanten]

Now that they're split off of the main component per #36 they're a lot easier to test :)

[Copilot]

Pull Request Overview

This PR adds comprehensive unit tests for the Vue split panel composables (useSizes, useResize, usePointer, useKeyboard, useGridTemplate) that were recently extracted from the main component. The tests improve code coverage and validate the behavior of these reusable functions.

• Adds test files for 5 composables covering size calculations, resize handling, pointer interactions, keyboard navigation, and grid template generation
• Updates existing test files to use data-testid attributes instead of CSS class selectors for more reliable test targeting
• Makes minSize parameter optional in the useSizes composable interface

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/vue-split-panel/tests/mounting.test.ts	Updates selectors to use data-testid attributes
packages/vue-split-panel/tests/collapse.test.ts	Updates selectors to use data-testid attributes
packages/vue-split-panel/src/composables/use-sizes.ts	Makes minSize parameter optional in interface
packages/vue-split-panel/src/composables/use-sizes.test.ts	Adds comprehensive tests for size calculations and conversions
packages/vue-split-panel/src/composables/use-resize.ts	Exports onResize callback for testing
packages/vue-split-panel/src/composables/use-resize.test.ts	Adds tests for resize observer behavior
packages/vue-split-panel/src/composables/use-pointer.test.ts	Adds tests for pointer interactions and double-click handling
packages/vue-split-panel/src/composables/use-keyboard.test.ts	Adds tests for keyboard navigation functionality
packages/vue-split-panel/src/composables/use-grid-template.test.ts	Adds tests for CSS grid template generation
packages/vue-split-panel/src/SplitPanel.vue	Adds data-testid attributes to DOM elements

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