Missing experimental session config: enableMcpApps

[krukow]

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Missing experimental config: enableMcpApps

Found during the GA upstream parity audit (Phase 1).

The Node.js SDK's session config exposes an experimental enableMcpApps opt-in
(nodejs/src/types.ts:1813, tag v1.0.0-beta.12). When true, the runtime reports
capabilities.ui.mcpApps on the create/resume response (types.ts:583,591,1801).

This SDK does not surface an equivalent config key.

Proposed resolution

Add an experimental :enable-mcp-apps session-config key (camelCased to enableMcpApps
on the wire), marked ^:experimental and documented as not part of the stable SDK
contract — consistent with how other experimental/CLI-adjacent options are handled. Or
explicitly omit it for GA and revisit when the upstream surface stabilizes.

Severity: post-ga (experimental upstream surface).