[Subtask] Support policies for view metadata objects

Describe the subtask

Support associating policies with VIEW metadata objects through the existing metadata object policy REST APIs.

Currently, VIEW exists in MetadataObject.Type, but it is excluded from PolicyManager.SUPPORTED_METADATA_OBJECT_TYPES_FOR_POLICIES. As a result, policy association requests for views are rejected with Cannot associate policies for unsupported metadata object type VIEW.

Implementation scope:

• Add VIEW to the supported metadata object types in PolicyManager.
• Clean up view policy relations when a view is dropped.
• Extend catalog and schema cascade cleanup for view policy relations across H2, MySQL, and PostgreSQL.
• Update docs/open-api/policies.yaml to include VIEW.
• Add unit and integration tests covering policy association, listing, retrieval, removal, direct view deletion, and cascade cleanup.

The completed view tag implementation in #11844 and #11897 can be used as a reference.

This task covers the server, core relational storage, OpenAPI, and their tests. Typed Java and Python client APIs can be handled in follow-up issues.

This task extends the current direct object-policy model and does not implement the policy-on-tag model proposed in #12176.

Parent issue

#604