Add frequently used utility extensions to string, enumerable, and date helpers

[Copilot]

Fills common gaps in the existing utility library with lightweight, idiomatic extension methods across three areas.

String (TextUtil)

• IsNullOrEmpty / IsNullOrWhiteSpace — extension-method shorthands
• ToIntOrDefault, ToDecimalOrDefault, ToDoubleOrDefault — safe-parse with fallback value
• Repeat(count) — repeat a string N times
• ToBase64 / FromBase64 — UTF-8 ↔ Base64, null-safe

Enumerable (EnumerableExtensions)

• IsNullOrEmpty<T>() — null-safe empty check
• ForEach<T>(Action<T>) — in-line side-effect iteration

Date (DateExtensions)

• StartOfDay — complement to existing EndOfDay
• StartOfMonth / EndOfMonth — first/last moment of the month (EndOfMonth reuses EndOfDay for consistency)
• IsWeekend / IsWeekday — day-type predicates
• AddWorkDays(int) — advance by N business days, skipping weekends; supports negative values

Usage example

// Safe parse
int qty = "42".ToIntOrDefault();          // 42
int bad = "abc".ToIntOrDefault(-1);       // -1

// Enumerable
items.ForEach(x => x.Process());
if (list.IsNullOrEmpty()) return;

// Date
var monthEnd  = DateTime.Today.EndOfMonth();   // last day 23:59:59
var nextBizDay = DateTime.Today.AddWorkDays(1);

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.