Improve error handling in SetControllerReference calls

[chrisguidry]

Summary

This PR addresses the error handling gaps identified in issue #161 by implementing proper error handling for SetControllerReference calls in the PrefectServer controller.

Problem

The helper functions prefectServerDeployment() and prefectServerService() were ignoring errors from SetControllerReference calls with TODO comments, which could lead to:

• Resources not being properly owned by their controllers
• Silent failures that are difficult to debug
• Potential runtime panics when invalid schemes are provided

Solution

• Modified function signatures to return errors
• Added nil scheme validation to prevent panics before calling SetControllerReference
• Implemented proper error propagation from helper functions to callers
• Added comprehensive error handling tests to verify proper behavior

Changes

• prefectServerDeployment() now returns (deployment, pvc, job, error) instead of (deployment, pvc, job)
• prefectServerService() now returns (service, error) instead of service
• All call sites updated to handle the new error return values
• Added 4 new test cases specifically for error handling scenarios

Test Coverage

• Baseline coverage: 56.3%
• Final coverage: 56.3% (maintained)
• All tests passing: ✅

Testing

The new tests verify that:

1. Helper functions properly return errors when scheme is nil
2. Error messages are descriptive and helpful
3. Errors are properly propagated through the reconciliation chain
4. Different configuration scenarios (ephemeral, SQLite, PostgreSQL) handle errors correctly

Fixes #161

Test plan

• All existing tests continue to pass
• New error handling tests validate proper error propagation
• Linters pass without issues
• Coverage remains stable

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