fix: handle empty and single-line output in PowerShell completion - #2497

Summary

Fixes #2481. The generated PowerShell completion script failed with runtime errors in three edge cases:

1. The completion command produced no output (e.g. the CLI crashed or returned nothing): $Out[-1].TrimStart(':') was called on a null value (Cannot index into a null array).
2. The completion command produced a single directive-only line: in newer PowerShell versions -OutVariable unwraps single-element results, so $Out[-1] returns a System.Char instead of a System.String and TrimStart is not available.
3. A completion value collided with the directive line: $Out = $Out | Where-Object { $_ -ne $Out[-1] } removed every line equal to the directive value, dropping legitimate completions (e.g. a completion whose value is :0).

The template in powershell_completions.go now:

• normalizes $Out into an array ([Array]$Directives = $Out) before reading the directive, so $null, a single string, or a multi-line result are all handled safely;
• parses the directive defensively and strips the last line by index with Select-Object -First, instead of filtering by value;
• returns early when there are no completions left after removing the directive, instead of indexing into $Out.

Tests

• go test ./... — passes (cobra, cobra/doc)
• go vet ./... — passes
• golangci-lint run --new-from-rev=origin/main --new ./... — 0 issues

Verification

Exercised the generated script in Windows PowerShell 5.1 against real binaries speaking the completion protocol (empty output, directive-only output, colliding directive/completion values, and a normal CLI built on cobra):

• empty output: old script crashed; new script completes with no errors
• directive-only output: old script surfaced a PSArgumentNullException on PS 5.1; new script completes with no errors
• colliding value: old script dropped the :0 completion; new script returns it
• normal completions (subcommands, ValidArgsFunction): behavior is unchanged

Fixes #2481