Request-builder: GraphQL layout (#291 Phase D follow-up)

[thomas-stegemann]

Why

Followup to #291. Phases A+B+C+E shipped (REST / gRPC / MCP / MQTT / WebSocket / SSE + streaming response pane). Phase D (GraphQL) was deferred to keep the initial #291 ship focused — the GraphQL layout has the most schema-discovery shape (Query/Mutation/Subscription + variables + selection-set picker) and warrants its own descriptor.

What's left

• GraphQL layout descriptor (requestBuilderLayout({ id: 'graphql', … })) registered via the rbLayouts registry that Hopp-bar: protocol picker (REST / gRPC / MQTT / WebSocket / SSE / GraphQL / MCP) with per-protocol sub-tab layout #291 introduced.
• Second-control: Operation-kind picker (Query / Mutation / Subscription).
• Sub-tabs per the Hopp-bar: protocol picker (REST / gRPC / MQTT / WebSocket / SSE / GraphQL / MCP) with per-protocol sub-tab layout #291 ticket table: Query · Variables · Header · Auth · Pre/Post-script · Variables.
• Reuse the existing GraphQL helpers in protocols.js (buildGraphQLDefaultQuery, wrapAsGraphQLRequest, buildGraphQLSelectionSet, graphqlSelections) — they already power the workbench's GraphQL flow.
• History verb formatter (_formatHistoryVerb in request-builder.js) already returns ps.operation || 'query' for the graphql slot; wire ps.operation through the layout.
• CSS for the GraphQL operation picker (mirror the MCP-kind picker palette in bowire.css).
• Subscription is a streaming case — should follow the same renderResponse override pattern that MQTT/WS/SSE use today.

Composes with

• Hopp-bar: protocol picker (REST / gRPC / MQTT / WebSocket / SSE / GraphQL / MCP) with per-protocol sub-tab layout #291 (parent ticket — the registry contract + the bar shell ship there)
• Pre-/post-scripts with a protocol-typed sandbox #126 scripts sandbox (Pre/Post run regardless of wire)
• The existing GraphQL plugin path (Kuestenlogik.Bowire.Protocol.GraphQL) via /api/invoke with protocol: 'graphql'

Acceptance

• Selecting GraphQL on the protocol dropdown swaps the bar shell to the GraphQL layout.
• Query / Variables editors are wired through the GraphQL request shape.
• Subscription operations open a streaming response pane and the Execute label cycles through Connect → Disconnect.
• No regression on REST / gRPC / MCP / MQTT / WebSocket / SSE.