ci: expand test matrix across platforms

[chiimagnus]

Motivation

• Run the project's test suite across macOS, Ubuntu and Windows runners to validate cross-platform Swift behavior.
• Ensure swift test runs properly on Windows by forcing the step to use the bash shell when needed.

Description

• Change runs-on to use the matrix value �${{ matrix.os }} and add a strategy.matrix.os containing macos-latest, ubuntu-latest, and windows-latest.
• Keep swift-actions/setup-swift@v2 with swift-version: '6.2' to maintain the Swift toolchain setup across platforms.
• Split the test step into two conditional steps so non-Windows platforms run swift test normally and Windows runs swift test with shell: bash.

Testing

• No automated tests were executed locally because this change only updates the CI workflow file.
• CI is expected to run swift test on each matrix entry (macOS, Ubuntu, Windows) when the workflow executes.

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Codex Task