Don't warn when JDK path from env var is not a directory

[Copilot]

When JI_JAVA_HOME (or other JDK path sources) points to a path that exists but is not a directory (e.g., a file or broken symlink), TryGetJdkInfo was propagating the ArgumentException from JdkInfo..ctor as a TraceLevel.Warning, producing noisy, misleading output in MSBuild logs.

Changes

• JdkInfo.TryGetJdkInfo: Add an early Directory.Exists guard. Paths that don't resolve to a directory are now silently skipped — logged only at Verbose — rather than emitting a Warning.

// Before: ArgumentException from ctor was caught and logged as Warning
internal static JdkInfo? TryGetJdkInfo(string path, ...)
{
    try { jdk = new JdkInfo(path, locator); }
    catch (Exception e) {
        logger(TraceLevel.Warning, ...);  // ← showed up in MSBuild output
    }
}

// After: non-directory paths are skipped before construction
if (!Directory.Exists(path)) {
    logger(TraceLevel.Verbose, $"Skipping JDK path `{path}`...");
    return null;
}

• New test GetKnownSystemJdkInfos_JiJavaHome_NonDirectoryPath_NoWarning: sets JI_JAVA_HOME to a temp file path and asserts no Warning is logged.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ArgumentException looking for JDKs</issue_title>
<issue_description>This happened in a test here:

• https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=5751716&view=ms.vss-test-web.build-test-results-tab&runId=33356848&resultId=100155&paneView=attachments
• Seems unrelated to the changes: [Xamarin.Android.Build.Tasks] update built-in .aotprofile's android#6745

23:21:22.063   1:7>/Users/runner/android-toolchain/dotnet/packs/Microsoft.Android.Sdk.Darwin/31.0.200-ci.pr.gh6745.77/tools/Xamarin.Android.Tooling.targets(69,5): warning : An exception occurred while validating the Java SDK installation in '/Users/runner/Library/Android/jdk-11' that was found while searching the paths from '$JI_JAVA_HOME'. Ensure that the Android section of the Visual Studio options has a valid Java SDK directory configured. To use a custom SDK path for a command line build, set the 'JavaSdkDirectory' MSBuild property to the custom path. Exception: Not a directory (Parameter 'homePath') [/Users/runner/work/1/a/TestRelease/02-14_22.59.11/temp/NothingToBindclass-parse/UnnamedProject.csproj]
                     System.ArgumentException: Not a directory (Parameter 'homePath')
                        at Xamarin.Android.Tools.JdkInfo..ctor(String homePath, String locator, Action`2 logger) in /Users/builder/azdo/_work/2/s/xamarin-android/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/JdkInfo.cs:line 57
                        at Xamarin.Android.Tools.JdkInfo..ctor(String homePath, String locator) in /Users/builder/azdo/_work/2/s/xamarin-android/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/JdkInfo.cs:line 97
                        at Xamarin.Android.Tools.JdkInfo.TryGetJdkInfo(String path, Action`2 logger, String locator) in /Users/builder/azdo/_work/2/s/xamarin-android/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/JdkInfo.cs:line 347 (TaskId:57)

The MSBuild log says $JI_JAVA_HOME is:

JI_JAVA_HOME = /Users/runner/Library/Android/jdk-11
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes ArgumentException looking for JDKs #154

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