fix: support extensionless file targets; use trailing separator for containers

Copilot review identified that the extension-only heuristic broke
extensionless file targets common on macOS/Linux (e.g. /usr/local/bin/terraform).

New branching logic:
- Trailing separator on Target => always a container (captured before
  GetUnresolvedProviderPathFromPSPath normalizes it away)
- No trailing separator + (has extension OR parent already exists) => file target
- Otherwise => container (created on Install)

This restores backward-compatible handling of extensionless file paths while
preserving the trailing-slash escape hatch for callers who want an
extensionless container target.

Update tests to use trailing slashes where container semantics are intended.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: HeyItsGilbert

PSDepend/PSDependScripts/FileDownload.ps1

@@ -77,6 +77,11 @@ Write-Verbose "Using URL: $URL"
 # Act on target path....
 $ToInstall = $False # Anti pattern
 
+# Capture trailing-separator intent before GetUnresolvedProviderPathFromPSPath normalizes it away;
+# a trailing separator is an explicit signal that the target is a container, not a file.
+$endsWithSeparator = $Target -and
+    $Target[-1] -in @([IO.Path]::DirectorySeparatorChar, [IO.Path]::AltDirectorySeparatorChar)
+
 # Resolve PSDrive paths (e.g. TestDrive:) and relative paths to absolute filesystem paths
 $Target = $PSCmdlet.GetUnresolvedProviderPathFromPSPath($Target)
 
@@ -92,8 +97,15 @@ if (Test-Path $Target -PathType Leaf) {
     }
     $PathToAdd = Split-Path $Target -Parent
 }
-elseif ([IO.Path]::GetExtension($Target) -and -not (Test-Path $Target -PathType Container)) {
-    # Target has a file extension — treat as a full destination file path
+elseif (
+    -not $endsWithSeparator -and
+    -not (Test-Path $Target -PathType Container) -and
+    ([IO.Path]::GetExtension($Target) -or (Test-Path $TargetParent))
+) {
+    # Treat as a full destination file path.
+    # Triggered when: has a file extension, OR parent already exists and caller gave no trailing separator.
+    # The trailing-separator check preserves extensionless binary targets (e.g. /usr/local/bin/terraform)
+    # while still allowing directory-like targets to be signalled with a trailing slash.
     $PathToAdd = $TargetParent
     if (-not (Test-Path $TargetParent)) {
         Write-Error "Could not find parent path [$TargetParent] for target [$Target]"
@@ -104,10 +116,10 @@ elseif ([IO.Path]::GetExtension($Target) -and -not (Test-Path $Target -PathType
     }
     $Path = $Target
     $ToInstall = $True
-    Write-Verbose "Target has extension, treating as file path [$Target]"
+    Write-Verbose "Treating as destination file path [$Target]"
 }
 else {
-    # No extension (or already a container) — treat target as a directory
+    # No extension (or already a container, or explicit trailing separator) — treat target as a directory
     Write-Verbose "[$Target] is a container, creating path to file"
     if (-not (Test-Path $Target) -and $PSDependAction -contains 'Install') {
         New-Item -ItemType Directory -Path $Target -Force | Out-Null

Tests/FileDownload.Type.Tests.ps1

@@ -73,8 +73,10 @@ Describe 'FileDownload script' -Skip:$SkipUnsupported {
     }
 
     It 'Creates a new directory and downloads into it when Target has no extension and does not exist' {
-        $newDir = Join-Path (New-Item 'TestDrive:/dl5base' -ItemType Directory -Force).FullName 'newcontainer'
-        $dep = New-PSDependFixture -DependencyName 'https://example.com/sample.dll' -DependencyType 'FileDownload' -Target $newDir
+        $base = (New-Item 'TestDrive:/dl5base' -ItemType Directory -Force).FullName
+        $newDir = Join-Path $base 'newcontainer'
+        # Trailing separator signals "this is a container, not an extensionless file"
+        $dep = New-PSDependFixture -DependencyName 'https://example.com/sample.dll' -DependencyType 'FileDownload' -Target "$newDir/"
         InModuleScope PSDepend -Parameters @{ Dep = $dep; ScriptPath = $script:ScriptPath; T = $newDir } {
             & $ScriptPath -Dependency $Dep
         }
@@ -100,7 +102,8 @@ Describe 'FileDownload script' -Skip:$SkipUnsupported {
         $baseDir = (New-Item 'TestDrive:/relbase' -ItemType Directory -Force).FullName
         Push-Location $baseDir
         try {
-            $dep = New-PSDependFixture -DependencyName 'https://example.com/sample.dll' -DependencyType 'FileDownload' -Target 'subdir'
+            # Trailing separator signals "this is a container, not an extensionless file"
+            $dep = New-PSDependFixture -DependencyName 'https://example.com/sample.dll' -DependencyType 'FileDownload' -Target 'subdir/'
             InModuleScope PSDepend -Parameters @{ Dep = $dep; ScriptPath = $script:ScriptPath } {
                 & $ScriptPath -Dependency $Dep
             }