fix(git-ext): honor explicit -Number 0 in PowerShell branch creation (#3412)

`-Number` defaults to 0, so the previous `-eq 0` / `-ne 0` checks could
not distinguish an unset flag from an explicit `-Number 0`: a user
requesting branch `000-...` was silently routed into auto-detection.
Switch both checks to `$PSBoundParameters.ContainsKey('Number')`, which
tests whether the flag was actually supplied — mirroring the bash twin's
empty-string sentinel (`[ -z "$BRANCH_NUMBER" ]` / `[ -n ... ]`).

Add a parity regression test to both TestCreateFeatureBash and
TestCreateFeaturePowerShell asserting `--number 0` / `-Number 0` yields
`000-zero`.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

extensions/git/scripts/powershell/create-new-feature-branch.ps1

@@ -446,7 +446,10 @@ if ($env:GIT_BRANCH_NAME) {
         $branchSuffix = Get-BranchName -Description $featureDesc
     }
 
-    if ($Timestamp -and $Number -ne 0) {
+    # Warn if -Number and -Timestamp are both specified. Use ContainsKey (not
+    # `-ne 0`) so an explicit `-Number 0` is also detected, matching the bash twin's
+    # `[ -n "$BRANCH_NUMBER" ]` check.
+    if ($Timestamp -and $PSBoundParameters.ContainsKey('Number')) {
         Write-Warning "[specify] Warning: -Number is ignored when -Timestamp is used"
         $Number = 0
     }
@@ -456,7 +459,10 @@ if ($env:GIT_BRANCH_NAME) {
         $branchName = New-BranchName -FeatureNum $featureNum -BranchSuffix $branchSuffix
     } else {
         $branchScopePrefix = Get-BranchScopePrefix -Template $branchTemplate -BranchSuffix $branchSuffix
-        if ($Number -eq 0) {
+        # Auto-detect the next number only when -Number was not supplied; an
+        # explicit value (including 0) is honored, matching the bash twin's
+        # `[ -z "$BRANCH_NUMBER" ]` check.
+        if (-not $PSBoundParameters.ContainsKey('Number')) {
             if ($DryRun -and $hasGit) {
                 $Number = Get-NextBranchNumber -SpecsDir $specsDir -SkipFetch -ScopePrefix $branchScopePrefix
             } elseif ($DryRun) {