fix: make copilot CLI path resolution cross-platform

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Eryx.AgentHost/Services/CopilotCliPathResolver.cs

@@ -8,6 +8,10 @@ internal static class CopilotCliPathResolver
     private const string CopilotCommandName = "copilot";
     private const string DefaultWindowsCommandProcessor = "cmd.exe";
     private const string DefaultWindowsPathExtensions = ".COM;.EXE;.BAT;.CMD";
+    private const char WindowsSearchPathSeparator = ';';
+    private const char UnixSearchPathSeparator = ':';
+    private const char WindowsDirectorySeparator = '\\';
+    private const char UnixDirectorySeparator = '/';
 
     private static readonly string[] BlockedCliEnvironmentPrefixes = ["BUN_", "COPILOT_", "ELECTRON_", "NODE_", "NPM_"];
 
@@ -125,7 +129,7 @@ internal static class CopilotCliPathResolver
         {
             foreach (string candidateName in GetCandidateFileNames(pathExtValue, isWindows))
             {
-                string candidatePath = Path.Combine(directory, candidateName);
+                string candidatePath = CombineSearchPath(directory, candidateName, isWindows);
                 if (fileExists(candidatePath))
                 {
                     return candidatePath;
@@ -145,7 +149,7 @@ internal static class CopilotCliPathResolver
 
         StringComparer comparer = isWindows ? StringComparer.OrdinalIgnoreCase : StringComparer.Ordinal;
         foreach (string directory in pathValue
-            .Split(Path.PathSeparator, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Split(GetSearchPathSeparator(isWindows), StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
             .Select(segment => segment.Trim('"'))
             .Where(segment => !string.IsNullOrWhiteSpace(segment))
             .Distinct(comparer))
@@ -184,6 +188,38 @@ internal static class CopilotCliPathResolver
         }
     }
 
+    private static char GetSearchPathSeparator(bool isWindows)
+    {
+        return isWindows ? WindowsSearchPathSeparator : UnixSearchPathSeparator;
+    }
+
+    private static string CombineSearchPath(string directory, string fileName, bool isWindows)
+    {
+        if (string.IsNullOrEmpty(directory))
+        {
+            return fileName;
+        }
+
+        if (EndsWithDirectorySeparator(directory, isWindows))
+        {
+            return directory + fileName;
+        }
+
+        return directory + GetDirectorySeparator(isWindows) + fileName;
+    }
+
+    private static bool EndsWithDirectorySeparator(string path, bool isWindows)
+    {
+        char lastCharacter = path[^1];
+        return lastCharacter == GetDirectorySeparator(isWindows)
+            || (isWindows && lastCharacter == UnixDirectorySeparator);
+    }
+
+    private static char GetDirectorySeparator(bool isWindows)
+    {
+        return isWindows ? WindowsDirectorySeparator : UnixDirectorySeparator;
+    }
+
     private static IEnumerable<KeyValuePair<string, string?>> GetCurrentEnvironmentVariables()
     {
         return Environment.GetEnvironmentVariables()

sidecar/tests/Eryx.AgentHost.Tests/CopilotCliPathResolverTests.cs

@@ -8,9 +8,10 @@ public sealed class CopilotCliPathResolverTests
     public void Resolve_UsesCopilotFromPath()
     {
         string copilotDirectory = @"C:\tools\copilot";
+        string expectedCliPath = @"C:\tools\copilot\copilot.exe";
         HashSet<string> existingFiles = new(StringComparer.OrdinalIgnoreCase)
         {
-            Path.Combine(copilotDirectory, "copilot.exe"),
+            expectedCliPath,
         };
 
         string? cliPath = CopilotCliPathResolver.Resolve(
@@ -19,16 +20,17 @@ public sealed class CopilotCliPathResolverTests
             isWindows: true,
             fileExists: existingFiles.Contains);
 
-        Assert.Equal(Path.Combine(copilotDirectory, "copilot.exe"), cliPath, ignoreCase: true);
+        Assert.Equal(expectedCliPath, cliPath, ignoreCase: true);
     }
 
     [Fact]
     public void Resolve_UsesDefaultWindowsExtensionsWhenPathExtMissing()
     {
         string copilotDirectory = @"C:\tools\copilot";
+        string expectedCliPath = @"C:\tools\copilot\copilot.cmd";
         HashSet<string> existingFiles = new(StringComparer.OrdinalIgnoreCase)
         {
-            Path.Combine(copilotDirectory, "copilot.cmd"),
+            expectedCliPath,
         };
 
         string? cliPath = CopilotCliPathResolver.Resolve(
@@ -37,7 +39,25 @@ public sealed class CopilotCliPathResolverTests
             isWindows: true,
             fileExists: existingFiles.Contains);
 
-        Assert.Equal(Path.Combine(copilotDirectory, "copilot.cmd"), cliPath, ignoreCase: true);
+        Assert.Equal(expectedCliPath, cliPath, ignoreCase: true);
+    }
+
+    [Fact]
+    public void Resolve_UsesCopilotFromPathOutsideWindows()
+    {
+        const string expectedCliPath = "/usr/local/bin/copilot";
+        HashSet<string> existingFiles = new(StringComparer.Ordinal)
+        {
+            expectedCliPath,
+        };
+
+        string? cliPath = CopilotCliPathResolver.Resolve(
+            pathValue: "/usr/bin:/usr/local/bin",
+            pathExtValue: null,
+            isWindows: false,
+            fileExists: existingFiles.Contains);
+
+        Assert.Equal(expectedCliPath, cliPath);
     }
 
     [Fact]