fix: remove stdlib log import from slice reconciler SetupWithManager

[mdryaan]

Description

controllers/slice/reconciler.go was importing the stdlib "log" package for a single log.Fatalf call inside SetupWithManager. This bypasses the structured logger used everywhere else in the project, writes to stderr in a different format, and calls os.Exit(1) without giving the controller-runtime manager a chance to run shutdown hooks.

Every other reconciler in this codebase returns errors from SetupWithManager — main.goalready handles them with the structured logger and os.Exit(1). This just makes the slice reconciler consistent with that pattern.

Changed log.Fatalf("Error creating label requirement: %v", err) to return fmt.Errorf("error creating label requirement: %w", err) and removed the "log" import. fmt was already imported so no new dependency is added.

Fixes #480

How Has This Been Tested?

• make fmt — no formatting changes
• make vet — no issues
• make build — builds cleanly

Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have ran go fmt
• I have updated the helm chart as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles.
• I have added all the required unit test cases.
• I have verified the E2E test cases with new code changes.
• I have added all the required E2E test cases.

Does this PR introduce a breaking change?