fix(executor): render tolerations as json

[jojosenthusiast]

Fixes #349

Reproduction

When executor.kubernetesJob.node.tolerations is provided as a YAML list, the executor ConfigMap rendered it as a Go-style map[...] string instead of JSON. The executor then fails to parse EXECUTOR_KUBERNETES_NODE_TOLERATIONS.

Fix summary

• Render non-string tolerations values with toJson | quote.
• Preserve existing string/default behavior for pre-encoded JSON strings and the default empty string.
• Add helm-unittest coverage for YAML list, pre-encoded JSON string, and default empty string inputs.
• Add the executor k8s chart unittest suite to the existing Buildkite unittest script.

Tests run

• git diff --check upstream/main...HEAD → pass
• sh -n scripts/ci/helm-unittest.sh → pass
• helm unittest -q charts/sourcegraph-executor/k8s → pass, 10/10 tests
• helm lint charts/sourcegraph-executor/k8s → pass, 1 chart linted, 0 failed (existing metadata-name warnings only)
• Static CI wiring check: .buildkite/pipeline.yaml runs scripts/ci/helm-unittest.sh, and the script now runs helm unittest -q charts/sourcegraph-executor/k8s

Risk

Low. Defaults remain unchanged and string inputs are preserved; YAML list/map inputs now render as JSON for the executor.

Non-goals

• No changes to executor chart defaults.
• No changes to sibling executor config fields with similar rendering patterns.