fix: parse pre-evaluation rules from Kubernetes specs

[ralph-bitgo]

What

• Correct the TestPreEvaluationRuleParse fixture to use the supported
  top-level spec.preEvaluationRules map shape.
• Add the JSON/YAML serialization tags required for the Kubernetes decoder to
  map preEvaluationRules into PrometheusServiceLevelSpec.

Why

• PR4's image replacement is independent of this code change, but its CI was
  blocked by the pre-existing TestPreEvaluationRuleParse failure. This
  minimal fix makes the test exercise the actual Kubernetes API shape and
  restores parsing of the pre-evaluation rule field, allowing PR4's image-only
  change to be validated without adding unrelated generated-artifact changes
  to either PR.

Test plan

• go test ./internal/k8sprometheus -run TestPreEvaluationRuleParse -count=1
• go test ./... -count=1
• git diff --check

Relationship to PR#4

PR#4 remains the sole PR containing
Sloth image replacement changes. PR#5 contains only the minimal parser/test
fix needed to unblock PR#4's unit-test CI. Merge PR#5 first, then validate and
merge PR#4 independently.

Ticket: INF-2854

[linear-code]

INF-2854

[Copilot]

Pull request overview

Fixes Kubernetes YAML decoding and the associated unit test so spec.preEvaluationRules is parsed into the internal Prometheus SLO model as intended.

Changes:

• Added JSON/YAML serialization tags for PrometheusServiceLevelSpec.PreEvaluationRules.
• Updated TestPreEvaluationRuleParse YAML fixture to use the supported top-level spec.preEvaluationRules shape.
• Adjusted expected model strings/metadata to match the decoded YAML block-scalar formatting and defaulted alert meta maps.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/kubernetes/api/sloth/v1/types.go	Adds struct tags so Kubernetes decoding maps preEvaluationRules onto the API type field.
internal/k8sprometheus/spec_test.go	Fixes the test fixture to the correct CRD shape and updates expectations to match decoded values.

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