Improve input sanitization and test coverage for name query param

[Copilot]

This PR hardens the input handling for the name query parameter to improve security, logging safety, and user experience.

Changes Made

Input Validation in CreateGreeting

• Whitespace trimming: Uses strings.TrimSpace() to remove leading/trailing whitespace
• Empty input handling: Returns "Hello, Guest" for empty strings after trimming
• Length limiting: Truncates names to 100 characters maximum to prevent abuse
• Control character sanitization: Strips newlines, tabs, and other control characters using unicode.IsControl()

Logging Safety in HTTP Handler

• Log injection prevention: Sanitizes the name parameter before logging to remove control characters
• Empty name logging: Shows "<empty>" in logs for empty names instead of blank entries

Comprehensive Test Coverage

Added 8 new test functions covering all edge cases:

• TestGreeting_WhitespaceOnly: Whitespace-only names → "Hello, Guest"
• TestGreeting_LongName: Names >100 chars → proper truncation
• TestGreeting_NewlineInjection: Control characters → safely removed
• TestGreeting_SpecialSymbols: Special symbols like !@# → preserved correctly
• TestGreeting_TrimWhitespace: Leading/trailing spaces → properly trimmed
• TestSanitizeControlChars: Unit test for control character removal
• TestSanitizeForLogging: Unit test for logging sanitization

Before/After Examples

Whitespace handling:

Input: "   " → Output: "Hello, Guest"
Input: " John " → Output: "Hello, John"

Security improvements:

Input: "Jane\nAdmin" → Output: "Hello, JaneAdmin" (newline removed)
Input: "User\x00" → Output: "Hello, User" (null byte removed)

Length limiting:

Input: 150 'a' characters → Output: "Hello, " + 100 'a' characters

All tests pass (10/10) and manual verification confirms the server correctly handles malicious inputs, logging is safe, and UX is improved.

Fixes #3.

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@copilot continue working on this PR

• Update the README - make it longer.

[Copilot]

@copilot continue working on this PR

• Update the README - make it longer.
  ...

Updated the README with comprehensive documentation. Expanded from 29 to 276 lines including project features, security enhancements, installation guides, API documentation, testing details, Docker deployment, CI/CD info, and contributing guidelines. Commit: 615fcee