…t file path completion (#515)

This PR addresses an issue where PowerShell argument completers would
fall back to file path completion when no valid argument matches were
found, leading to irrelevant file and folder suggestions.

## Problem

When using argument completion with GitHub PowerShell module commands,
if no matches were found for the typed input, PowerShell would default
to showing file and folder completions from the current directory. This
created a confusing user experience where typing something like:

```powershell
Get-GitHubLicense -Name invalidlicense<TAB>
```

Would show local files and folders instead of no completions, suggesting
invalid options to the user.

## Solution

Updated all 34 argument completers across 14 files to explicitly return
`$null` when no matches are found. The fix follows this pattern:

**Before:**
```powershell
Get-GitHubLicense @params | Where-Object { $_.Name -like $pattern } | ForEach-Object {
    [System.Management.Automation.CompletionResult]::new($_.Name, $_.Name, 'ParameterValue', $_.Name)
}
```

**After:**
```powershell
$filteredOptions = Get-GitHubLicense @params | Where-Object { $_.Name -like $pattern }
if (-not $filteredOptions) {
    return $null
}
$filteredOptions | ForEach-Object {
    [System.Management.Automation.CompletionResult]::new($_.Name, $_.Name, 'ParameterValue', $_.Name)
}
```

## Result

- ✅ When valid matches exist: Shows appropriate completions as before
- ✅ When no matches exist: No completions shown (cursor stays in place)
- ❌ Previously: Would show irrelevant file/folder completions when no
matches found

This provides a cleaner, more intuitive completion experience that
doesn't suggest invalid options to users.

## Files Modified

All completer files throughout the module structure:
- Core completers in `/src/completers.ps1`
- Function-specific completers in
`/src/functions/public/*/completers.ps1`

Fixes #514.

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…518)

## Description

This pull request updates the `Secrets.Tests.ps1` test suite to improve
how the `Get-GitHubPublicKey` command is tested for different
organization plans and makes a minor formatting change to a log message.

Test logic improvements for organization plan handling:

* Updated the `Get-GitHubPublicKey -Type codespaces` test to
conditionally expect a throw for organizations on the 'free' plan, and
to run the usual public key retrieval for other plans. This ensures the
test behaves correctly based on the organization's plan type.

## Type of change

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [ ] 📖 [Docs]
- [x] 🪲 [Fix]
- [ ] 🩹 [Patch]
- [ ] ⚠️ [Security fix]
- [ ] 🚀 [Feature]
- [ ] 🌟 [Breaking change]

## Checklist

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas