madengine v2 refactor(k8s) secrets handling, launcher mixin, and template updates

[coketaste]

• Add k8s_secrets helpers for image pull / runtime secrets with configurable strategies; add unit tests.
• Introduce KubernetesLauncherMixin for torchrun and related launcher command generation; wire into kubernetes deployment.
• Update K8s Job/ConfigMap templates, k8s defaults preset, and kubernetes.py.
• Refresh configuration/deployment docs and k8s-configs README.
• Adjust tests (conftest, dummy credential fixture) for new behavior.
• Fix CLI merge of --additional-context with --additional-context-file when the CLI value uses Python dict syntax (fallback after JSON parse failure).

[Copilot]

Pull request overview

Refactors madengine’s Kubernetes (v2) deployment to improve secret handling, launcher command generation, and manifest templating—aiming to keep credentials out of ConfigMaps while supporting multiple secret strategies and more launchers.

Changes:

• Add k8s_secrets helpers + defaults to create/resolve image-pull/runtime Secrets (with ConfigMap size preflight + unit tests).
• Introduce KubernetesLauncherMixin and wire it into KubernetesDeployment; update K8s Job/ConfigMap templates to mount credentials via Secret when applicable.
• Improve CLI --additional-context merge behavior (JSON first, fallback to Python dict syntax) and refresh K8s-related docs.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/unit/test_k8s_secrets.py	Adds unit tests for secret strategy merging, secret ref resolution, and ConfigMap payload size estimation.
tests/conftest.py	Ensures src/ is on sys.path for consistent test imports.
src/madengine/deployment/templates/kubernetes/job.yaml.j2	Adds TTL support, conditional imagePullSecrets, and mounts runtime credentials from Secret when configured; gates privileged profiling securityContext.
src/madengine/deployment/templates/kubernetes/configmap.yaml.j2	Stops embedding credential.json unless explicitly requested.
src/madengine/deployment/presets/k8s/defaults.json	Adds defaults for TTL, privileged profiling toggle, and k8s.secrets configuration.
src/madengine/deployment/kubernetes_launcher_mixin.py	New mixin providing launcher command string generators (torchrun/deepspeed/torchtitan/vLLM/SGLang/etc.).
src/madengine/deployment/kubernetes.py	Integrates secrets strategy + ConfigMap size preflight, uses launcher mixin, updates GPU plugin validation messaging, and propagates imagePullSecrets to collector pod.
src/madengine/deployment/k8s_secrets.py	New module for creating/deleting secrets from credential.json, resolving secret refs, and estimating ConfigMap payload sizes.
src/madengine/cli/commands/run.py	Fixes merging of --additional-context with file-based context by attempting JSON parse first, then ast.literal_eval.
examples/k8s-configs/README.md	Documents new Secrets behavior and multi-node DNS differences across launcher types.
docs/deployment.md	Documents Secrets behavior, manifest validation workflow, and clarifies TTL cleanup behavior.
docs/configuration.md	Updates K8s preset docs and documents new k8s.secrets/TTL/profiling fields.

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