feat: add update_campaign_goal

[elmuby]

Closes #83

This PR enables creators to reduce their campaign goal while strictly preventing any increase to mitigate rug pull risks.

• Added InvalidGoalUpdate error
• Implemented update_campaign_goal with:
  • Creator-only access
  • Active campaign validation
  • new_goal <= current_goal (no increases)
  • new_goal >= total_raised (cannot go below raised amount)
• Added campaign_goal_updated event

Tests

Added tests to verify:

• Successful goal reduction
• Increase attempt reverts with InvalidGoalUpdate
• Cannot reduce below total_raised
• Cannot update expired campaign

All tests passing ✅

[elmuby]

Kindly review my pr @Akshola00

[Akshola00]

Kindly review my pr @Akshola00

Thanks
Pls remove test snapshots as they are added in the gitignore