feat: split KoP schema registry namespace

[mattisonchao]

Motivation

• Oxia metadata deployments currently let KoP schema registry fall back to the broker metadata namespace when oxiaSchemaRegistryUrl is not set.
• KoP reads oxiaSchemaRegistryUrl, while Pulsar schema storage is controlled separately by schemaRegistryStorageClassName and oxiaSchemaStorageUrl.

Modifications

• Add a kafka-schema Oxia namespace helper and render a matching OxiaNamespace.
• Configure KoP schema registry to use kafka-schema via PULSAR_PREFIX_oxiaSchemaRegistryUrl.
• Replace the oxiaBasedSystemTopic chart block with explicit broker custom config so Kafka schema registry, group offsets, producer state, and topic-policy settings are visible in chart output.
• Do not force schemaRegistryStorageClassName; the current default 3.3.3.3 image does not include io.streamnative.pulsar.schema.OxiaSchemaStorageFactory. Pulsar schema metadata therefore remains on the default schema storage path unless users provide an image/plugin and set that storage class explicitly.

Testing

• git diff --check
• helm lint charts/sn-platform
• helm lint charts/sn-platform-slim
• helm template split-schema charts/sn-platform --namespace pulsar --set pulsar_metadata.provider=oxia
• helm template split-schema-slim charts/sn-platform-slim --namespace pulsar --set pulsar_metadata.provider=oxia
• Fresh local kind deployment on context kind-matt with charts/sn-platform-slim, Oxia metadata, 2 brokers, and existing sn-license secret. Verified all pods became ready and rendered config uses /kafka-schema for oxiaSchemaRegistryUrl.

[github-actions]

@mattisonchao:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)