Build initial MCP SSH server

tests/McpSsh.Tests/DefaultSshKeyResolverTests.cs

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+using McpSsh.Server.Ssh;
+
+namespace McpSsh.Tests;
+
+public sealed class DefaultSshKeyResolverTests
+{
+    [Fact]
+    public void ResolveDefaultKeyPath_ReturnsFirstExistingDefaultKey()
+    {
+        var fileSystem = new FakeFileSystem("/home/test/.ssh/id_ecdsa", "/home/test/.ssh/id_rsa");
+        var resolver = new DefaultSshKeyResolver(fileSystem, "/home/test/.ssh");
+
+        var path = resolver.ResolveKeyPath(null);
+
+        Assert.Equal("/home/test/.ssh/id_ecdsa", path);
+    }
+
+    [Fact]
+    public void ResolveDefaultKeyPath_ThrowsWhenNoDefaultKeyExists()
+    {
+        var resolver = new DefaultSshKeyResolver(new FakeFileSystem(), "/home/test/.ssh");
+
+        var ex = Assert.Throws<SshToolException>(() => resolver.ResolveKeyPath(null));
+
+        Assert.Equal("ssh_key_not_found", ex.ErrorCode);
+    }
+
+    [Fact]
+    public void ResolveKeyPath_ReturnsExplicitKeyWhenItExists()
+    {
+        var resolver = new DefaultSshKeyResolver(new FakeFileSystem("/keys/deploy_ed25519"), "/home/test/.ssh");
+
+        var path = resolver.ResolveKeyPath("/keys/deploy_ed25519");
+
+        Assert.Equal("/keys/deploy_ed25519", path);
+    }
+
+    [Fact]
+    public void ResolveKeyPath_ThrowsWhenExplicitKeyDoesNotExist()
+    {
+        var resolver = new DefaultSshKeyResolver(new FakeFileSystem(), "/home/test/.ssh");
+
+        var ex = Assert.Throws<SshToolException>(() => resolver.ResolveKeyPath("/keys/missing"));
+
+        Assert.Equal("ssh_key_not_found", ex.ErrorCode);
+    }
+
+    private sealed class FakeFileSystem : IFileSystem
+    {
+        private readonly HashSet<string> _paths;
+
+        public FakeFileSystem(params string[] paths)
+        {
+            _paths = paths.ToHashSet(StringComparer.Ordinal);
+        }
+
+        public bool FileExists(string path) => _paths.Contains(path);
+    }
+}