feat(test_demo): Add validation for command-line options

Adds error handling for unknown or invalid command-line options:
- Unknown options (e.g., --invalid) print error and help, then exit(1)
- Missing arguments (e.g., --resolution without WxH) print error and help
- Invalid format (e.g., --resolution abc) print error and help

Error handling:
- Prints specific error message to stderr
- Shows full help text for reference
- Exits with status code 1 (error)
- --help still exits with status code 0 (success)

Examples of new behavior:
  $ test_demo --unknown
  Error: Unknown option '--unknown'
  [help text displayed]

  $ test_demo --resolution
  Error: --resolution requires an argument (e.g., 1024x768)
  [help text displayed]

  $ test_demo --resolution abc
  Error: Invalid resolution format 'abc' (expected WxH, e.g., 1024x768)
  [help text displayed]

This prevents silent failures and helps users discover correct usage.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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