⚙️ [Maintenance]: Hard-lock Pester test dependency to 5.8.0 by GUID (#42)

Hard-locks the Pester test dependency to an exact version pinned by
module GUID, replacing the `RequiredVersion = '5.7.1'` pin that broke
test discovery when the runner installed Pester 5.8.0.

```powershell
#Requires -Modules @{ ModuleName = 'Pester'; RequiredVersion = '5.8.0'; GUID = 'a699dea5-2c73-4616-a270-1f7abb777e71' }
```

## Why 5.8.0 + GUID
- **Exact version** = reproducible and supply-chain-safe (no silent
drift to a newer or compromised release).
- **GUID** pins module identity (anti-name-squat). Both the version and
the GUID are enforced by PowerShell at discovery time (verified
locally).
- **No version comment** — unlike a GitHub Actions `@<sha>`,
`RequiredVersion` already states the version in plain text, and
`#Requires` rejects a GUID without a version key.

Files updated: 1 test file(s).

Fixes #41.

> [!NOTE]
> This pin fully holds once `Invoke-Pester` installs the pinned version
instead of the latest (today it installs newest, so a future Pester
release would re-break exact pins). Tracked in
PSModule/Invoke-Pester#68. Part of the dependency-management epic
PSModule/Process-PSModule#356.

Author: MariusStorhaug

tests/AST.Tests.ps1

@@ -1,4 +1,4 @@
-#Requires -Modules @{ ModuleName = 'Pester'; RequiredVersion = '5.7.1' }
+#Requires -Modules @{ ModuleName = 'Pester'; RequiredVersion = '5.8.0'; GUID = 'a699dea5-2c73-4616-a270-1f7abb777e71' }
 
 Describe 'Core' {
     Context "Function: 'Get-AstScript'" {