fix(extensions/git): reject negative -Number in create-new-feature-branch.ps1 (#3538)

The bash and Python twins validate --number against ^[0-9]+$ and reject a
negative value with 'Error: --number must be a non-negative integer'. The
PowerShell twin declares the parameter as [long]$Number, so PowerShell binds
'-5' as -5 instead of rejecting it. That value then formats via '{0:000}' to
'-005' and yields a branch name starting with a dash, which git refuses (refs
cannot begin with '-') — a confusing late failure instead of the twins' clear
early error.

Guard for $Number -lt 0 up front (before the description check, matching the
bash twin's parse-time validation order) and emit the identical error. An
explicit -Number 0 is still honored, preserving the #3412 fix.

Add matching negative-number parity tests to the bash and PowerShell
create-feature suites, mirroring the existing test_explicit_number_zero_is_honored
pair. Same PowerShell-parity bug class as #3412.

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

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

@@ -41,6 +41,16 @@ if ($Help) {
     exit 0
 }
 
+# -Number is [long], so PowerShell binds "-5" as -5 rather than rejecting it
+# the way the bash/Python twins do (`^[0-9]+$`). A negative value would format
+# via '{0:000}' to e.g. "-005" and produce a branch name starting with "-",
+# which git refuses (refs cannot begin with a dash). Reject it here, before the
+# description check, matching the bash twin's parse-time validation order.
+if ($Number -lt 0) {
+    Write-Error 'Error: --number must be a non-negative integer'
+    exit 1
+}
+
 if (-not $FeatureDescription -or $FeatureDescription.Count -eq 0) {
     Write-Error "Usage: ./create-new-feature-branch.ps1 [-Json] [-DryRun] [-AllowExistingBranch] [-ShortName <name>] [-Number N] [-Timestamp] <feature description>"
     exit 1