feat: add optional Icon field to Goal model and implement tests

[JKASONGA1]

Summary

This PR adds icon support to the Goal model and covers the GetGoalsForUser route with unit tests.

Changes

• Added optional Icon field (string?) to the Goal model in Goal.cs
• Updated appsettings.json and Program.cs to connect to MongoDB Atlas
• Updated CommBank.csproj to target net9.0
• Implemented GetForUser unit test in GoalControllerTests.cs
• Updated CommBank.Tests.csproj to target net9.0

Testing

• All 11 unit tests pass (dotnet test)
• API tested with Postman - GET /api/goal returns goals with icon field

[gitguardian]

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GitGuardian id	GitGuardian status	Secret	Commit	Filename
-	-	MongoDB Credentials	fcf5c30	CommBank-Server/appsettings.json	View secret

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