Address code review feedback on input validation and API correctness

[Copilot]

Addresses all review comments from the initial PR, fixing exception handling inconsistencies, unsafe context requirements, enum flag collisions, and input validation gaps.

Changes

• Library.cs: Split exception types per .NET guidelines - ArgumentNullException only when pattern is null, ArgumentException for empty/whitespace patterns
• Program.cs: Replaced sizeof(IntPtr) with IntPtr.Size to eliminate unsafe context requirement
• ProcessAccessRights.cs: Removed PROCESS_GET_CONTEXT and PROCESS_SET_CONTEXT (thread rights incorrectly included in process rights enum, causing flag collision at 0x1000)
• Wildcard.cs: Added explicit hex character validation via ParseNibble() helper method, preventing cryptic FormatException from downstream Convert.ToInt32

// Before: ambiguous exception for whitespace-only patterns
if (string.IsNullOrWhiteSpace(pattern))
    throw new ArgumentNullException(nameof(pattern), "...");

// After: correct exception types per .NET guidelines
if (pattern is null)
    throw new ArgumentNullException(nameof(pattern), "...");
if (patternTokens.Length == 0)
    throw new ArgumentException("The search pattern is empty or contains only whitespace.", nameof(pattern));

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