Add OTEL k8s taints integration tests

[louisall]

Summary

Adds integration tests for the k8staintsprocessor which decorates node telemetry with k8s.node.taint.<key> resource attributes.

Tests added

• TestTaintsAppearOnDaemonSetMetrics — verifies taint attributes on DaemonSet metrics (local_mode=true)
• TestTaintsEmptyValueSkipped — verifies empty-value taints are NOT emitted (Granite rejects blank attribute values)
• TestTaintsAbsentOnUntaintedNodes — verifies untainted nodes have no k8s.node.taint.* attributes
• TestTaintsAppearOnClusterScraperMetrics — verifies cluster-scraper (local_mode=false) sees taints
• TestTaintsMultipleOnSameNode — verifies multiple taints appear together on the same node

Terraform changes

Adds null_resource.apply_test_taints that applies two test taints (ci-test.example.com/dedicated=gpu and ci-test.example.com/team=ml) to the first node in the standard node group before tests run.

Dependencies

This PR will not pass CI until the following upstream PRs are merged and deployed:

1. amazon-contributing/opentelemetry-collector-contrib #509 — k8staintsprocessor implementation
2. Agent PR (not yet opened) — registers k8staintsprocessor in components.go + bumps go.mod SHA (blocked on Integration test for Prometheus EMF Pipeline #509 merging)
3. aws-observability/helm-charts #300 — wires k8staints processor into all Container Insights pipelines

Validated

All 5 tests pass against a live EKS cluster with a hand-built agent containing the processor.