Fix Windows test compatibility for ProcessBuilder usage

[edburns]

Fixes #65 .

On Windows, Java's ProcessBuilder cannot directly run shell wrappers like npx (installed as npx.cmd) or Unix commands like cat. Tests that used these commands failed with "Cannot run program" errors. Additionally, Unix-style paths like "/nonexistent/copilot" are not absolute on Windows, causing assertThrows(IOException) tests to pass unexpectedly when CliServerManager wrapped them with "cmd /c".

Changes:

• CapiProxy: use "cmd /c npx" on Windows to launch the test harness
• CliServerManagerTest: replace "cat" with cross-platform dummy process; use a platform-appropriate nonexistent absolute path so IOException is thrown on all platforms
• JsonRpcClientTest: replace "cat" with cross-platform dummy process

All changes use runtime os.name detection and preserve existing behavior on Linux and macOS. Full test suite passes on all platforms (556 tests, 0 failures, 0 errors).

Resolves #65

[Copilot]

Pull request overview

Improves Windows compatibility for tests that spawn external processes via ProcessBuilder, avoiding failures due to Windows-specific executable resolution (.cmd) and Unix-only commands/paths.

Changes:

• Update CapiProxy to start the test harness via cmd /c npx ... on Windows.
• Update unit tests to avoid Unix-only process commands and to use a Windows-absolute nonexistent CLI path.
• Preserve existing behavior on Linux/macOS via runtime os.name detection.

Show a summary per file

File	Description
src/test/java/com/github/copilot/sdk/JsonRpcClientTest.java	Switches dummy process creation to use a Windows-compatible command.
src/test/java/com/github/copilot/sdk/CliServerManagerTest.java	Uses a Windows-absolute nonexistent CLI path and updates stdio dummy process creation.
src/test/java/com/github/copilot/sdk/CapiProxy.java	Launches npx tsx server.ts through cmd /c on Windows to handle npx.cmd.

Copilot's findings

Comments suppressed due to low confidence (1)

src/test/java/com/github/copilot/sdk/JsonRpcClientTest.java:156

• Same issue as above: the Windows dummy process (cmd /c type NUL) can exit immediately, so this test may not actually cover the “process becomes dead after being alive” scenario. Consider switching the Windows branch to a command that blocks on stdin so the process is reliably alive until you destroy it in the test.

        boolean isWindows = System.getProperty("os.name").toLowerCase().contains("win");
        Process proc = isWindows
                ? new ProcessBuilder("cmd", "/c", "type", "NUL").start()
                : new ProcessBuilder("cat").start();
        var client = JsonRpcClient.fromProcess(proc);
        proc.destroy();
        proc.waitFor(5, TimeUnit.SECONDS);
        assertFalse(client.isConnected());

• Files reviewed: 3/3 changed files
• Comments generated: 2