Decompile-params: Resolve bicep file path relative to current directory (#15986)

Resolves #13785.
###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/Azure/bicep/pull/15986)

---------

Co-authored-by: Levi Muriuki <[email protected]>

Author: levimatheri

src/Bicep.Cli.IntegrationTests/CliScenarioTests.cs

@@ -1,9 +1,11 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+using Bicep.Core.FileSystem;
 using Bicep.Core.UnitTests.Assertions;
 using Bicep.Core.UnitTests.Utils;
 using FluentAssertions;
+using FluentAssertions.Execution;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
 namespace Bicep.Cli.IntegrationTests
@@ -187,5 +189,51 @@ param roleAssignmentName string
 
                 """);
         }
+
+        [TestMethod]
+        public async Task Test_Issue13785()
+        {
+            var paramFile =
+              """
+              {
+                "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentParameters.json#",
+                "contentVersion": "1.0.0.0",
+                "parameters": {
+                  "foo": {
+                    "value": "bar"
+                  }
+                }
+              }
+              """;
+            var expectedOutput =
+                """
+                using '../main.bicep'
+
+                param foo = 'bar'
+
+                """;
+
+            var (jsonPath, bicepparamPath) = Setup(TestContext, paramFile);
+            Directory.SetCurrentDirectory(Path.GetDirectoryName(jsonPath)!);
+            var (output, _, result) = await Bicep("decompile-params", jsonPath, "--bicep-file", "../main.bicep");
+
+            using (new AssertionScope())
+            {
+                output.Should().BeEmpty();
+                result.Should().Be(0);
+                File.ReadAllText(bicepparamPath).Should().BeEquivalentToIgnoringNewlines(expectedOutput);
+            }
+        }
+
+        private static (string jsonPath, string bicepparamPath) Setup(TestContext context, string template, string? inputFile = null, string? outputDir = null)
+        {
+            var jsonPath = FileHelper.SaveResultFile(context, inputFile ?? "param.json", template);
+
+            var bicepparamPath = outputDir is null 
+              ? PathHelper.GetDefaultDecompileparamOutputPath(jsonPath) 
+              : FileHelper.GetResultFilePath(context, outputDir);
+
+            return (jsonPath, bicepparamPath);
+        }
     }
 }

src/Bicep.Cli/Commands/DecompileParamsCommand.cs

@@ -37,7 +37,7 @@ public int Run(DecompileParamsArguments args)
             var inputUri = PathHelper.FilePathToFileUrl(PathHelper.ResolvePath(args.InputFile));
             var outputPath = PathHelper.ResolveDefaultOutputPath(inputUri.LocalPath, args.OutputDir, args.OutputFile, PathHelper.GetDefaultDecompileparamOutputPath);
             var outputUri = PathHelper.FilePathToFileUrl(outputPath);
-            var bicepUri = args.BicepFilePath is { } ? PathHelper.FilePathToFileUrl(args.BicepFilePath) : null;
+            var bicepUri = args.BicepFilePath is { } ? PathHelper.FilePathToFileUrl(PathHelper.ResolvePath(args.BicepFilePath)) : null;
 
             try
             {