Improve input sanitization and test coverage for name query param

[nofarblue]

📝 Description
The current app (CreateGreeting and the HTTP handler) does not handle edge cases in the name query parameter. This can result in:

• Unhelpful or inconsistent logs when name is empty, whitespace-only, or malformed
• Potential for log injection or hard-to-read logs due to special characters (e.g., newlines)
• Poor UX when names contain only whitespace or very long strings

We want to harden the input handling to make the app safer and the logs cleaner.

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✅ Acceptance Criteria

• Input validation (in CreateGreeting):

  • Trim leading/trailing whitespace
  • Return "Hello, Guest" if input is empty after trim
  • Limit accepted name length to 100 characters
  • Optionally strip or escape newline/control characters

• Logging safety (in HTTP handler):

  • Escape or sanitize name before logging (e.g., remove newline/control chars)

• Test coverage:

  • Name is empty → "Hello, Guest"
  • Name is only whitespace → "Hello, Guest"
  • Name includes newline or control characters
  • Name with special symbols (e.g., Jane!@#)
  • Very long name (>100 characters) → name should be truncated or rejected

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🧪 Sample Unit Test Cases to Add

func TestGreeting_WhitespaceOnly(t *testing.T) { ... }
func TestGreeting_LongName(t *testing.T) { ... }
func TestGreeting_NewlineInjection(t *testing.T) { ... }

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🧩 Notes
This is a minimal, self-contained enhancement ideal for evaluating Copilot’s issue assignment capabilities. It:

• Involves logic + test changes
• Doesn’t require any external APIs
• Keeps scope limited but realistic
• Touches areas where Copilot should excel (sanitization, string ops, tests)