Gate vector index creation by platform capabilities

Summary

Gate the Index Management Vector create experience by the connected platform and verified Azure DocumentDB vector-index capabilities.

PR #732 currently exposes the Azure DocumentDB cosmosSearch Vector tab on every connection. Until gating is implemented, unsupported platforms or tiers can complete the form and discover the limitation only when creation fails. This limitation is accepted for the initial release and should be addressed in the upcoming patch release.

Proposed scope

• Add an authoritative host-side platform/capability result for Index Management.
• Show the Azure DocumentDB Vector tab only when appropriate, or disable it with an actionable unsupported reason.
• Gate direct create and Playground/Shell preparation through the same host-side capability rule.
• Distinguish platform support from tier/version/algorithm support where the server exposes that detail.
• Keep server errors actionable when a capability cannot be known before submission.

Related work

• PR #732 adds Index Management and Azure DocumentDB vector index creation.
• #815 tracks MongoDB Atlas detection and a separate Search Index tab. The platform switch should eventually select the Azure DocumentDB Vector experience or Atlas Search Index experience without conflating their resource models.

Acceptance criteria

• Unsupported platforms are not presented with a fully enabled Azure DocumentDB Vector create flow.
• Capability enforcement lives on the extension host and applies consistently to direct create and prepared commands.
• The UI explains why Vector is unavailable and what the user can do next.
• Focused tests cover supported, unsupported, and unknown-capability states.

Start by reviewing PR #732 and the existing Index Management Vector flow, then trace the extension-host path for direct creation and Playground/Shell preparation. Implement the shared capability decision and focused tests for supported, unsupported, and unknown states; done means the UI is actionable and unsupported platforms cannot submit the fully enabled flow.