fix(driver-kubernetes): propagate log_level as OPENSHELL_LOG_LEVEL env var

[mesutoezdil]

Summary

The Kubernetes driver was inserting logLevel and environment fields at the Sandbox CR spec level inside sandbox_to_k8s_spec. Neither field exists in the agents.x-k8s.io/v1alpha1 CRD schema. Kubernetes accepts them on unstructured writes and the controller drops them silently, so log_level never reached the sandbox pod.

Changes:

• Remove the dead CR-level insertions of logLevel and environment
• Build a merged environment map that includes OPENSHELL_LOG_LEVEL when spec.log_level is set, and pass it to sandbox_template_to_k8s so the value flows through build_env_list into the pod container spec
• Add a unit test verifying OPENSHELL_LOG_LEVEL appears in the container env and that logLevel is absent from the CR spec

Related Issue

Fixes #1265

Testing

• mise run pre-commit passes
• Unit test added: log_level_propagates_as_env_var_to_sandbox_pod
• All 29 unit tests pass

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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[TaylorMutch]

Validated locally on a fresh k3d cluster via mise run helm:k3s:create + mise run helm:skaffold:run against this branch (commit 64fd96ab).

Findings

• Unit test driver::tests::log_level_propagates_as_env_var_to_sandbox_pod passes.
• End-to-end positive case: CreateSandbox with spec.log_level="debug" produces a Sandbox CR where the agent container's env array contains OPENSHELL_LOG_LEVEL=debug, and the legacy spec.logLevel / spec.environment fields at the CR root are no longer written. The downstream pod inherits the env var ({"name":"OPENSHELL_LOG_LEVEL","value":"debug"}).
• End-to-end negative case: CreateSandbox with an empty log_level produces no OPENSHELL_LOG_LEVEL entry in the container env, leaving only the standard supervisor variables — confirming the filter(|s| !s.log_level.is_empty()) guard.
• The new spec_pod_env helper is correctly threaded through both the template-present and the default-template branches of sandbox_to_k8s_spec, so behavior is consistent regardless of whether the caller supplied a template.

LGTM.