Everything-is-an-app: validate values.custom.yaml against vendor values.schema.json

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## Parent

linode/apl-core#3515

## What to build

Server-side and in-editor validation of `values.custom.yaml` against the vendor chart's own `values.schema.json`, replacing the curated per-app schema for configuration validation.

End-to-end behavior:
- When a chart ships `charts//values.schema.json`, a write to that app's `values.custom.yaml` is validated against it before the file is committed to the values repo. Invalid input is rejected with actionable errors.
- When a chart ships no vendor schema, the write is accepted as-is with no validation gate (errors surface later at `helm template`/ArgoCD sync).
- The same validation is applied in the console's raw YAML/JSON editor when a vendor schema exists, so users get in-editor feedback where possible.
- Validation is a pure function (chart name + custom-values content in, accept / reject-with-errors out) so it is unit-testable without git or Kubernetes.

Use `cert-manager` (ships a vendor schema) and one schema-less core chart as fixtures to cover both the validated and no-gate paths.

## Acceptance criteria

- [ ] A pure validation function rejects `values.custom.yaml` content that violates the chart's vendor `values.schema.json`, with useful error messages.
- [ ] The same function accepts any content for a chart with no vendor schema (no gate).
- [ ] Server-side write path (apl-api parity) refuses invalid custom values before commit; schema-less charts are accepted.
- [ ] Console raw editor surfaces live validation errors when a vendor schema exists for the edited chart.
- [ ] Unit tests use real vendor-schema fixtures for at least one schema-bearing chart (cert-manager) and one schema-less chart.

## Blocked by

- linode/apl-core#3571

Start by reviewing the blocker, linode/apl-core#3571, then trace the server-side values.custom.yaml write path and the console raw YAML/JSON editor. Implement and test pure validation using cert-manager and a schema-less core chart, confirming invalid values are rejected, schema-less values are accepted, and both integration paths show the expected behavior.

Written by the indexing model from the issue text.