Merge pull request #1398 from mattcave/fix/spf-circular-reference

# Fix circular SPF reference detection (A→B→A loops)

1. Fixed `a:` Mechanism Domain Resolution

- Now correctly extracts and resolves the domain specified in the directive (e.g., `a:sub.example.com`)
- Previously always resolved `$Name` instead of the directive's target domain, returning incorrect IP addresses

2. Fixed MX Mechanism A Record Lookup

- Corrected non-Windows path that was querying `$Name` instead of each `$MXRecord` when resolving A records
- Previously returned wrong IP addresses for MX hosts

3. Improved DNS Server Parameter Handling

- Now only passes `-NameServer`/`-Server` parameter when explicitly provided by user
- Uses parameter splatting to conditionally build DNS query parameters
- Prevents passing empty/null values to DNS cmdlets

4. Better Error Handling

- Changed error returns from strings to proper `Write-Error` and `Write-Warning` cmdlets
- Maintains consistent output type contract `([OutputType([SPFRecord[]])])`
- Added `-ErrorAction SilentlyContinue` to DNS lookups to gracefully handle failures

5. Parameter Validation

- Added `[ValidateNotNullOrEmpty()]` to `$Name` and `$Server` parameters
- Prevents unnecessary DNS calls with invalid input

6. Improved Variable Naming

- Used distinct variable names (`$MXDNSRecords`, `$ADNSRecords`) to prevent variable overwriting in nested loops
- Improves code clarity and prevents data loss

7. Documentation

- Completed comment-based help section
- Moved comment-based help header into the function block to align with best practices

Author: SamErde