[Helm] Allow setting extra env vars on the LMCache cache server container

[pujitha24]

[Helm] Allow setting extra env vars on the LMCache cache server container

FIX #923 (link existing issues this PR will resolve)

The LMCache cache server Deployment template had no env: block at all, so
there was no way to pass any environment variable to the cache-server
container through Helm values. Issue #923 reports the cache server getting
OOMKilled, and several commenters converged on the workaround of setting
LMCACHE_CACHE_POLICY / LMCACHE_MAX_LOCAL_CPU_SIZE (LMCache's own eviction
knobs) — but nobody could actually do that through this chart.

This PR does not pick an eviction policy or fix OOM behavior by default.
It only adds a generic, empty-by-default cacheserverSpec.env values field
(mirroring the existing env pattern used for routerSpec.env /
$container.env elsewhere in the chart) so users can self-configure LMCache's
eviction env vars, or any other env var, themselves. Default rendering is
byte-for-byte unchanged when cacheserverSpec.env is left unset.

Also updated helm/values.schema.json and helm/README.md to document the
new field, and extended the existing helm/tests/cacheserver_test.yaml suite
to cover it.

Validation

• helm unittest helm -f 'tests/cacheserver_test.yaml': 2/2 pass.
• helm unittest helm (full suite): 142 passed, 2 failed. Both failures
  (tests/deployment-vllm-multi_test.yaml asserts[3];
  tests/ray-cluster_test.yaml asserts[1..3]) are pre-existing and
  unrelated to this change — reproduced identically with this diff stashed
  out against unmodified main.
• helm lint helm: passes (only pre-existing unrelated warnings).
• helm template with no cacheserverSpec.env set renders the container
  with no env: key at all (unchanged default behavior); with
  --set-json 'cacheserverSpec.env=[{"name":"LMCACHE_CACHE_POLICY","value":"LRU"}]'
  it renders the env block correctly.
• Installed the helm-values-schema-json plugin and ran the exact
  helm-schema pre-commit hook this repo wires up in
  .pre-commit-config.yaml — it passed with zero additional diff, confirming
  values.schema.json is byte-identical to the tool's own generated output.
• Ran the repo's non-manual pre-commit hooks (check-json, check-yaml,
  end-of-file-fixer, trailing-whitespace, markdownlint, codespell) on all
  changed files: all passed.
• Could not run this repo's live-cluster "Functionality test for helm
  chart" GitHub Actions jobs (self-hosted minikube runners) — not available
  in this sandbox. Mitigated by the default-rendering check above: since this
  change is purely additive behind an empty-by-default list, it cannot affect
  any existing tests/assets/values-*.yaml file used by that workflow.
• Base branch CI (Functionality test for helm chart on main) is currently
  green.

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[gemini-code-assist]

Code Review

This pull request introduces support for configuring extra environment variables for the cache server container via the cacheserverSpec.env parameter. It updates the Helm chart templates, values schema, default values, and documentation, and adds corresponding unit tests to verify the environment variables are correctly injected. There are no review comments, and I have no feedback to provide.

[pujitha24]

Since it has been open a week without a look, @YuhanLiu11 as noted in the PR template — this is rebased and green, happy to adjust anything that would help review.