V2: add session form service #34973

Summary

Track the V2 session form service that provides a shared form/request lifecycle for session-scoped user input.

Scope

• Add the core V2 form service and durable/event lifecycle.
• Expose the protocol/server/client surface needed by TUI/app consumers.
• Support reply/cancel flows and pending form state.
• Provide the base primitive for follow-up integrations such as MCP elicitation.
• Keep concurrency/settlement behavior aligned with permission/question services; follow-up concurrency hardening is tracked separately in fix(core): serialize V2 prompt settlement around event publication #34853.

Implementation

PR: #34855

Follow-ups

• fix(core): serialize V2 prompt settlement around event publication #34853 — serialize V2 prompt settlement around event publication.
• [FEATURE]: Port MCP lifecycle/status/connect/disconnect APIs to V2 #34435 — MCP lifecycle/status and elicitation APIs; MCP elicitation should use this form service after it lands.
• convert the auth plugin prompts to use form shape, consider adding a condition based logic flow #34950 — convert auth/plugin prompts to shared form shape.