feat: add icon support to Goal model with unit tests

[alwaysprince05]

Summary

This PR adds icon support to the CommBank backend and extends test coverage.

Changes

Goal Model (Task 1)

• Added optional Icon field (string?) to Goal.cs
• Icons persist to MongoDB

Tests (Task 4)

• Implemented GetForUser test in GoalControllerTests.cs
• Asserts result not null, each goal is a Goal type, each goal has expected UserId
• All 11 tests pass

Testing

dotnet test → 11 passed, 0 failed

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

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🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
-	-	MongoDB Credentials	a85b835	CommBank-Server/Secrets.json	View secret

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