GitOps for Kibana content: discover sessions, Vega, annotations and Maps endpoints

**What the feature is (as Title)**
GitOps for Kibana Content: Discover Sessions, Vega, Annotations & Maps Endpoints

**Value proposition**
Extends dashboard-as-code coverage beyond core dashboards and visualizations to the remaining content types teams actually build with — saved Discover sessions, Vega charts, annotations, and Maps — so an entire Kibana workspace, not just part of it, can be version-controlled, automated, and promoted through environments. Markdown, Tags and the Links panel already have their own endpoints.

**Expected outcome**
Customers can manage the full set of Kibana content as code: defining it in git, reviewing changes via PR, and deploying through CI/CD, with no gaps that force a fallback to manual UI edits or export/import workarounds. This closes the last mile of the GitOps story started with the Dashboards & Visualizations APIs GA, and matters most to platform teams, MSPs, and partners running many environments or tenants that need consistent, auditable, reproducible content everywhere.

- Maps endpoints
- Discover sessions endpoints
- Vega endpoints
- Annotation endpoints

**Optional: Key user stories / use cases**

Start by reviewing the existing Dashboards & Visualizations APIs, then investigate how Kibana represents Maps, Discover sessions, Vega charts, and annotations. The issue names no files or tests, so the initial work is to identify the relevant API entry points and define the endpoint scope. Done means all four content types can be managed through GitOps workflows without manual export/import.

Written by the indexing model from the issue text.