fix: render ModelConfig TLS in Helm chart

[andreivince]

Closes #1991.

The chart-generated default ModelConfig already rendered provider, model, API key secret, defaultHeaders, and provider-specific config, but it dropped providers.<name>.tls. That meant the CRD supported ModelConfig TLS settings, while users installing through the chart could not pass them into the default ModelConfig.

This adds the missing TLS passthrough in templates/modelconfig.yaml and covers it with a Helm unit test. I also added a small commented example in values.yaml so the supported shape is easier to find.

I verified this with:

• Added the Helm regression test first and confirmed it failed on missing spec.tls.*
• helm unittest -f 'tests/modelconfig_test.yaml' helm/kagent
• make helm-test
• helm lint helm/kagent
• helm lint helm/kagent-crds
• Exact CI Helm v3.18.0 against the kagent, querydoc, and grafana-mcp chart unit tests
• Render checks for all built-in provider defaults with TLS set
• kubeconform against the rendered chart and CRDs
• Temporary kind cluster with CRDs installed and a server-side Helm dry run using TLS values

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[andreivince]

:)

[andreivince]

Pretty sure gotta re run the build, had the same issue my last PR