fix: prevent invalid input in builder mode numeric fields

[mustalk]

Resolves input validation issues in Builder mode where users could input invalid characters in numeric fields. On real devices, users could type minus signs despite coordinates being non-negative. On emulators, users could type any characters including letters and symbols. Implements real-time input filtering and enhanced validation logic.

Issues Fixed

• Fixes bug: builder mode accepts invalid characters in numeric input fields #9
• Users can no longer type minus signs on real devices
• Users can no longer type letters/symbols on emulators
• Clear error message differentiation between format and value errors
• Real-time feedback prevents invalid input rather than catching it later

Changes Made

Input Filtering (Prevention)

• Real-time character filtering in FieldUpdateHandler
• Numeric fields only accept digits: Plateau Width/Height, Rover Start X/Y
• No minus signs allowed for coordinate fields (coordinates are non-negative by design)
• Works consistently across real devices and emulators

Enhanced Validation (Detection)

• Improved validation logic in ValidationHandler
• Clear error message differentiation:
  • "Must be a valid number" for invalid format (letters, symbols)
  • "Must be a positive number" for zero values (plateau dimensions)
  • "Must be a non-negative number" for negative values (coordinates)

Comprehensive Testing

• Input filtering test coverage: Verify invalid characters are filtered
• Validation test updates: Account for new filtering behavior
• Edge case handling: Empty strings, mixed input, etc.

User Experience Improvements

Before Fix

• Real devices: Users could type "-" in coordinate fields
• Emulators: Users could type "ads" or any characters in numeric fields
• No immediate feedback
• Confusing error messages
• Errors only appeared after focus loss or execution

After Fix

• Real-time filtering - Invalid characters can't be entered
• Immediate visual feedback - Only valid digits appear
• Clear error messages - Distinguish between format and value errors
• Consistent behavior across device types

Testing

• Unit Tests: All existing tests pass
• New Tests: Input filtering behavior coverage
• Edge Cases: Empty input, mixed characters, boundary values
• Cross-platform: Works on both real devices and emulators

Manual Testing

• Try typing minus signs on real device numeric fields > Filtered out in real-time
• Try typing letters on emulator > Filtered out in real-time
• Try typing special characters > Filtered out in real-time
• Valid numeric input > Works as expected
• Mission execution with valid input > Successful
• Error messages > Clear and contextual

Impact

• User Experience: Improved input consistency across devices
• Data Integrity: Guaranteed valid numeric input
• Performance: No impact - filtering is lightweight