Fix HTTP 422 error in github_organization_settings resource

Fixes #2305

This commit resolves the HTTP 422 error that was occurring when using the
github_organization_settings resource, particularly with Enterprise Managed User (EMU)
organizations.

## Root Cause
The issue was caused by:
1. Unnecessary Organizations.Edit() call with nil parameters
2. Unconditional sending of all fields including empty strings for optional fields
3. Incorrect handling of boolean false values using d.GetOk() instead of d.GetOkExists()

## Changes Made

### Core Fixes
- Replace Organizations.Edit(ctx, org, nil) with Organizations.Get() to safely retrieve organization plan
- Create buildOrganizationSettings helper function that conditionally builds settings struct
- Use d.GetOkExists() for boolean fields to properly handle false values
- Only send explicitly configured fields to avoid API rejections

### Enterprise Support
- Properly handle Enterprise-specific fields like members_can_create_internal_repositories
- Detect enterprise organizations using plan name and conditionally include enterprise fields

### Test Coverage
- Add comprehensive test cases for all parameter types:
  * String fields (8 parameters)
  * Security boolean fields (6 parameters)
  * Repository creation fields (5 parameters)
  * Other boolean fields (3 parameters)
  * Enum fields (1 parameter)
- Add specific tests for boolean false value handling
- Add tests for complex multi-parameter configurations
- Add tests for enterprise-specific functionality

## Testing
- Manually tested with EMU organization
- Verified all parameter types work correctly
- Confirmed boolean false values are properly sent to API
- Tested complex configurations with multiple field types
- All existing and new tests pass

## Impact
- Resolves HTTP 422 errors for all organization types
- Enables proper management of EMU organization settings
- Maintains backward compatibility
- Improves API payload efficiency by only sending configured fields

Author: netflash