[Bug]: Add guard/null checks in HandleBars helpers & extensions

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: Several helpers cast/index without guards, so bad inputs throw NullReferenceException/IndexOutOfRangeException instead of a meaningful HandlebarsException; one error message is also inaccurate.
• Affected files: ECoreNetto.HandleBars/StringHelper.cs:47,59 (cast-then-use without null check), ECoreNetto.HandleBars/GeneralizationHelper.cs:71 (ESuperTypes[0] after an Any() check), ECoreNetto.HandleBars/StructuralFeatureHelper.cs:148 (message says "two arguments" but checks for one), ECoreNetto.Extensions/StringExtensions.cs:50 (words[0] without empty check)
• Proposed solution: Add explicit argument/null/empty validation with descriptive messages; fix the inaccurate "two arguments" message to match the actual check.
• Acceptance criteria:
  • Helpers throw descriptive HandlebarsException/ArgumentException on invalid input rather than low-level exceptions.
  • The argument-count error message matches the validation.
  • Tests cover the invalid-input paths.